Memory system

ABSTRACT

A memory system according to an embodiment includes a semiconductor memory, and a memory controller. The semiconductor memory comprises memory cells and word lines. Each of the word lines is connected to the memory cells. The memory controller executes a patrol operation including a read operation of the semiconductor memory. The word lines are classified into one of first and second groups. The memory controller executes patrol operations in which the word lines are respectively selected in a first patrol period and, in a second patrol period subsequent to the first patrol period, executes a patrol operation in which the word line included in the first group is selected and omits a patrol operation in which the word line included in the second group is selected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application NO. 2018-172913, filed Sep. 14, 2018, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments relate to a memory system.

BACKGROUND

A NAND-type flash memory which can nonvolatilely store data has beenknown.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration example of a memorysystem according to a first embodiment;

FIG. 2 is a block diagram showing a configuration example of a NANDpackage and a NAND interface circuit in the memory system according tothe first embodiment;

FIG. 3 is a block diagram showing a configuration example of a NAND-typeflash memory included in the NAND package included in the memory systemaccording to the first embodiment;

FIG. 4 is a circuit diagram showing an example of a circuitconfiguration of a memory cell array included in the NAND-type flashmemory in the first embodiment;

FIG. 5 is a plan view showing an example of a planar layout of thememory cell array included in the NAND-type flash memory in the firstembodiment;

FIG. 6 is a cross-sectional view showing an example of a cross-sectionalstructure of the memory cell array included in the NAND-type flashmemory in the first embodiment;

FIG. 7 is a cross-sectional view showing an example of a cross-sectionalstructure of a memory pillar included in the memory cell array includedin the NAND-type flash memory in the first embodiment;

FIG. 8 is a circuit diagram showing an example of a circuitconfiguration of a row decoder module included in the NAND-type flashmemory in the first embodiment;

FIG. 9 is a circuit diagram showing an example of a circuitconfiguration of a sense amplifier module included in the NAND-typeflash memory in the first embodiment;

FIG. 10 is a circuit diagram showing an example of a circuitconfiguration a sense amplifier unit included in the sense amplifiermodule included in the NAND-type flash memory in the first embodiment;

FIG. 11 is a diagram showing an example of threshold distribution, readvoltage, and verify voltage of a memory cell transistor in the memorysystem according to the first embodiment;

FIG. 12 is a threshold distribution chart showing an example of the readvoltage used in tracking read in the NAND-type flash memory in the firstembodiment and two adjacent threshold distributions;

FIG. 13 is a timing chart showing an example of tracking read in thememory system according to the first embodiment;

FIG. 14 is a timing chart showing an example of shift reading in thememory system according to the first embodiment;

FIG. 15 is a diagram showing an example of a command sequence in patroloperation of the memory system according to the first embodiment;

FIG. 16 is a table showing an example of processing order of the patroloperation in the memory system according to the first embodiment;

FIG. 17 is a timing chart showing an example of the patrol operation ofthe memory system according to the first embodiment;

FIG. 18 is a table showing an example of processing order of the patroloperation in the memory system according to the first embodiment;

FIG. 19 is a timing chart showing an example of the patrol operation inthe memory system according to the first embodiment;

FIG. 20 is a timing chart showing an example of patrol operation of amemory system according to a variation of the first embodiment;

FIG. 21 is a diagram showing an example of a command sequence in patroloperation of a memory system according to a second embodiment;

FIGS. 22 and 23 are diagrams showing an example of a timing chart in thepatrol operation of the memory system according to the secondembodiment;

FIG. 24 is a timing chart showing an example of an execution cycle ofpatrol operation in a memory system according to a comparative exampleof a third embodiment;

FIG. 25 is a timing chart showing an example of the execution cycle ofthe patrol operation in the memory system according to the thirdembodiment;

FIG. 26 is a graph showing an example of a change in read voltage afterwriting to a memory cell transistor in a memory system according to afourth embodiment;

FIG. 27 is a timing chart showing an example of the execution cycle ofthe patrol operation in the memory system according to the fourthembodiment;

FIG. 28 is a table showing an example of a tracking target state inabbreviated tracking read of a memory system according to a fifthembodiment;

FIG. 29 is a timing chart showing an example of abbreviated trackingread in the memory system according to the fifth embodiment;

FIG. 30 is a timing chart showing an example of an execution cycle ofpatrol operation in the memory system according to the fifth embodiment;

FIG. 31 is a graph showing an example of a temperature dependence ofdata retention characteristics and a patrol criteria of a memory celltransistor in a memory system according to a sixth embodiment;

FIG. 32 is a table showing an example of a patrol management parameterin patrol operation of the memory system according to the sixthembodiment;

FIG. 33 is a table showing an example of a patrol management parameterin patrol operation of a memory system according to a seventhembodiment; and

FIG. 34 is a timing chart showing an example of an execution cycle ofthe patrol operation in the memory system according to the seventhembodiment.

DETAILED DESCRIPTION

A memory system according to an embodiment includes a semiconductormemory, and a memory controller. The semiconductor memory comprises aplurality of memory cells connected in series and a plurality of wordlines. Each of the plurality of word lines is connected to each of thememory cells. The memory controller executes a patrol operationincluding a read operation of the semiconductor memory. The word linesare classified into one of a first group and a second group based on anaddress of the word line. The memory controller executes a plurality ofpatrol operations in which the word lines are respectively selected in afirst patrol period and, in a second patrol period subsequent to thefirst patrol period, executes a patrol operation in which the word lineincluded in the first group is selected and omits a patrol operation inwhich the word line included in the second group is selected.

Hereinafter, embodiments will be described with, reference to thedrawings. The embodiments to be described below exemplify devices andmethods for embodying the technical concepts of the invention. Thedrawings are schematic or conceptual, and the dimensions, ratios, andthe like in the respective drawings are not necessarily identical tothose in reality. The technical idea of the present invention is notspecified by the shapes, structures, and layouts of the constituentparts.

In the following explanation, the same reference numerals denoteconstituent elements having almost the same functions and arrangements.A number just after a character constituting a reference numeral isreferred to by the reference numeral containing the same character andis used for distinguishing the components having a similarconfiguration. A character just after a number constituting a referencenumeral is referred to by the reference numeral containing the samenumber and is used for distinguishing the components having a similarconfiguration. When the components indicated by the reference numeralscontaining the same character or number do not need to be distinguishedfrom each other, the components are referred to by the reference numeralcontaining only a character or number.

[1] First Embodiment

Hereinafter, a memory system 1 in a first embodiment will be described.

[1-1] Configuration

[1-1-1] Overall Configuration of Memory System 1

The memory system 1 according to the first embodiment is, for example, asolid state drive (SSD) and can nonvolatilely hold data. The memorysystem 1 is connected to an external host device 2, for example, and canexecute various operations in accordance with a command from the hostdevice 2.

FIG. 1 shows a configuration example of the memory system 1 according tothe first embodiment. As shown in FIG. 1, the memory system 1 accordingto the first embodiment includes, for example, NAND packages PKG0 andPKG1, a memory controller 20, and a dynamic random access memory (DRAM)30.

Each of the NAND packages PKG0 and PKG1 includes a plurality ofNAND-type flash memories. Details of the configuration of the NANDpackage PKG will be described later.

The memory controller 20 is, for example, system on chip (SoC). Thememory controller 20 issues instructions for reading, writing, erasingor the like to each of the NAND packages PKG0 and PKG1, in response toan instruction from the host device 2, for example.

The memory controller 20 includes, for example, a central processingunit (CPU) 21, a random access memory (RAM) 22, a host interface circuit23, an error correction code (ECC) circuit 24, a temperature sensor 25,a NAND interface circuit 26, a DRAM interface circuit 27, and a timer28.

The CPU 21 controls the overall operation of the memory controller 20.For example, the CPU 21 issues a write command in response to a writeinstruction received from the host device 2. The CPU 21 can executepatrol operation based on a count of a timer 28. Details of the patroloperation will be described later. Further, the CPU 21 executes variousprocesses for managing a memory space of the NAND package PKG, such aswear leveling.

The RAM 22 is a volatile memory such as static random access memory(SRAM), for example. The RAM 22 is used as a work area of the CPU 21,and holds, for example, a firmware for managing the NAND package PKG,various management tables, and the like. The management table includes,for example, a look up table (LUT) in which a correction value of a readvoltage is recorded.

The host interface circuit 23 is connected to the host device 2 via ahost bus and controls transfer of data, commands, and addresses betweenthe memory controller 20 and the host device 2. For example, the hostinterface circuit 23 may support communication interface standards suchas serial advanced technology attachment (SATA), serial attached SCSI(SAS), and PCI Express (PCIe) (registered trademark).

The ECC circuit 24 executes error correction processing of data. Duringwrite operation, the ECC circuit 24 generates a parity based on writedata received from the host device 2 and attaches the generated parityto the write data. During read operation, the ECC circuit 24 generates asyndrome based on read data received from the NAND package PKG anddetects and corrects an error in the read data based on the generatedsyndrome.

The temperature sensor 25 measures the system temperature of the memorysystem 1. The measured temperature is referred to by the CPU 21, forexample, and is used in the patrol operation to be described later. Thetemperature sensor 25 may not be included in the memory controller 20and may be externally connected to the memory controller 20 in thememory system 1. Further, the temperature sensor 25 may be incorporatedin a NAND-type flash memory 10 to be described later. In this case, thememory controller 20 can acquire temperature information from theNAND-type flash memory 10 by inputting a command to the NAND-type flashmemory 10.

The NAND interface circuit 26 controls transfer of data, commands, andaddresses between the memory controller 20 and the NAND package PKG. TheNAND interface circuit 26 supports a NAND interface standard.

The DRAM interface circuit 27 is connected to the DRAM 30 and governscommunication between the memory controller 20 and the DRAM 30. The DRAMinterface circuit 27 supports a DRAM interface standard.

The timer 28 can measure a time related to various operations of thememory system 1 and an elapsed time since data is written in the memorycell. The timer 28 may not be included in the memory controller 20 andmay be externally connected to the memory controller 20 in the memorysystem 1.

The DRAM 30 is a volatile memory capable of temporarily storing data andis used as an external storage area of the memory controller 20. Forexample, the DRAM 30 temporarily stores the write data received from thehost device 2. The DRAM 30 may be incorporated in the memory controller20.

FIG. 2 shows a configuration example of the NAND packages PKG0 and PKG1and the NAND interface circuit 26 in the memory system 1 according tothe first embodiment. As shown in FIG. 2, each of the NAND packages PKG0and PKG1 includes, for example, NAND-type flash memories 10A, 10B, 10C,and 10D. The NAND interface circuit 26 includes, for example, channelcontrollers CC0, CC1, CC2 and CC3.

The NAND-type flash memory 10 can nonvolatilely store data. Theconfiguration of the NAND-type flash memory 10 will be described later.

Each of the channel controllers CC0 to CC3 supports the NAND interfacestandard. The channel controllers CC0 to CC3 are connected to bus linesCh0 to Ch3, respectively. Each of the bus lines Ch0 to Ch3 is used fortransmitting and receiving a signal based on the NAND interfacestandard.

Further, each of the bus lines Ch0 to Ch3 is connected to a plurality ofthe NAND-type flash memories 10. That is, each of the channelcontrollers CC0 to CC3 is connected to the plurality of NAND-type flashmemories 10 via the corresponding bus line Ch. Specifically, the channelcontroller CC0 is connected to the NAND-type flash memories 10A and 10Bin the NAND package PKG0 via the bus line Ch0. The channel controllerCC1 is connected to the NAND-type flash memories 10C and 10D in the NANDpackage PKG0 via the bus line Ch1. The channel controller CC2 isconnected to the NAND-type flash memories 10A and 10B in the NANDpackage PKG1 via the bus line Ch2. The channel controller CC3 isconnected to the NAND-type flash memories 10C and 10D in the NANDpackage PKG1 via the bus line Ch3.

As described above, each of the NAND packages PKG includes the pluralityof NAND-type flash memories 10 connected to different channelcontrollers CC. For example, the NAND-type flash memories 10 connectedto a common bus line, Ch are allocated to different banks BNK.

The bank BNK is defined by a set of the NAND-type flash memories 10connected to different bus lines Ch, for example. A set of the NANDpackages PKG0 and PKG1 includes, for example, banks BNK0 and BNK1. Forexample, the bank BNK0 includes the NAND-type flash memories 10A and 10Cin the NAND package PKG0 and the NAND-type flash memories 10A and 10C inthe NAND package PKG0. The bank BNK1 includes the NAND-type flashmemories 10B and 10D in the NAND package PKG0 and the NAND-type flashmemories 10B and 10D in the NAND package PKG1.

It should be noted that the configuration of the memory system 1described above is merely an example, and the present invention is notlimited thereto. For example, the number of the NAND packages PKG andthe number of the volatile memories included in the memory system 1 mayeach be designed in any number. The memory system 1 may include othervolatile memories in place of the DRAM 30. Each function of the memorycontroller 20 may be realized by a dedicated hardware circuit or may berealized by executing a firmware with the CPU 21.

The number of the NAND-type flash memories 10 included in the NANDpackage PKG may be designed in any number. The number of the channelcontrollers CC connected to each of the NAND packages PKG may bedesigned in any number. The number of the banks BNK may be appropriatelychanged based on the number of the NAND-type flash memories 10 includedin each of the NAND packages PKG and the number of the channelcontrollers CC connected to the NAND package PKG.

[1-1-2] Configuration of NAND-Type Flash Memory 10

FIG. 3 shows a configuration example of the NAND-type flash memory 10according to the first embodiment. As shown in FIG. 3, the NAND-typeflash memory 10 includes, for example, a memory cell array 11, aninput/output circuit 12, a register set 13, a logic controller 14, asequencer 15, a ready/busy controller 16, a voltage generator 17, a rowdecoder module 18, and a sense amplifier module 19.

The memory cell array 11 includes blocks BLK0 to BLKn (n is an integerof 1 or more). The block BLK is a group of memory cells capable ofnonvolatilely holding data, and is used as a unit of use of data, forexample. Each memory cell is associated with a bit line BL and a wordline WL.

The input/output circuit 12 transmits and receives input/output signalsI/O1 to I/O8 of, for example, 8-bit width to and from the memorycontroller 20. The input/output signal I/O may include, for example,data DAT, address information ADD, command CMD, and the like. Forexample, in the write operation, the input/output circuit 12 transfersthe write data DAT received from the memory controller 20 to the senseamplifier module 19. On the other hand, in the read operation, theinput/output circuit 12 transmits the read data DAT transferred from thesense amplifier module 19 to the memory controller 20.

The register set 13 includes a status register 13A, an address register13B, and a command register 13C. The status register 13A holds statusinformation STS. The status information STS may include a status of theNAND-type flash memory 10 and a parameter related to the read operation.The status information STS is transferred to the input/output circuit 12under control of the sequencer 15, for example, and then output to thememory controller 20. The address register 13B holds the addressinformation ADD transferred from the input/output circuit 12. Theaddress information ADD may include, for example, a block address, apage address, a column address, and the like. The address informationADD is transferred to the voltage generator 17, the row decoder module18, and the sense amplifier module 19, for example, under the control ofthe sequencer 15. The command register 13C holds the command CMDtransferred from the input/output circuit 12. The command CMD isreferred to by the sequencer 15.

The logic controller 14 controls each of the input/output circuit 12 andthe sequencer 15 based on a control signal received from the memorycontroller 20. As such a control signal, for example, a chip enablesignal/CE, a command latch enable signal CLE, an address latch enablesignal ALE, a write enable signal/WE, a read enable signal/RE, and awrite protect signal/WP are used.

The chip enable signal/CE is a signal for enabling the NAND-type flashmemory 10. The command latch enable signal CLE is a signal for notifyingthe input/output circuit 12 that the received input/output signal I/O isthe command CMD. The address latch enable signal ALE is a signal fornotifying the input/output circuit 12 that the received input/outputsignal I/O is the address information ADD. The write enable signal/WE isa signal for instructing the input/output circuit 12 to input theinput/output signal I/O. The read enable/RE is a signal instructing theinput/output circuit 12 to output the input/output signal I/O. The writeprotect signal/WP is a signal for placing the NAND-type flash memory 10in a protected state when a power supply is turned on and off.

The sequencer 15 controls the overall operation of the NAND-type flashmemory 10. For example, the sequencer 15 controls the voltage generator17, the row decoder module 18, the sense amplifier module 19, and thelike based on the address information ADD and the command CMD held inthe register set 13 to execute various operations.

The ready/busy controller 16 generates a ready/busy signal RBn based onan operation state of the sequencer 15. The ready/busy signal RBn is asignal for notifying the memory controller 20 whether the NAND-typeflash memory 10 is in a ready state to accept a command from the memorycontroller 20 or in a busy state in which no command is accepted.

The voltage generator 17 generates a desired voltage under the controlof the sequencer 15. Then, the voltage generator 17 supplies thegenerated voltage to the memory cell array 11, the row decoder module18, the sense amplifier module 19, and the like.

The row decoder module 18 is connected to a word line or the likeprovided in the memory cell array 11. For example, the row decodermodule 18 selects the block BLK executing various operations based onthe block address. Then, the row decoder module 18 transfers the voltagesupplied from the voltage generator 17 to various wires in the selectedblock BLK.

The sense amplifier module 19 is connected to the bit line provided inthe memory cell array 11. In the read operation, the sense amplifiermodule 19 reads the data DAT from the memory cell array 11 and transfersthe read data DAT to the input/output circuit 12. During the writeoperation, the sense amplifier module 19 applies a desired voltage tothe bit line based on the data DAT received from the input/outputcircuit 12.

[1-1-3] Configuration of Memory Cell Array 11

(Circuit Configuration of Memory Cell Array 11)

FIG. 4 is an example of a circuit configuration of the memory cell array11 included in the NAND-type flash memory in the first embodiment, inwhich the single block BLK is extracted and shown. The block BLKincludes, for example, four string units SU0 to SU3, as shown in FIG. 4.Each of the string units SU includes a plurality of NAND strings NS.

The plurality of NAND strings NS are associated with bit lines BL0 toBLm (m is an integer of 1 or more), respectively. Each of the NANDstrings NS includes, for example, memory cell transistors MT0 to MT15,dummy transistors LDT and UDT, and select transistors ST1 and ST2.

The memory cell transistor MT includes a control gate and a chargeaccumulation layer and nonvolatilely stores data. Each of the dummytransistors LDT and UDT has the same configuration as, for example, thememory cell transistor MT. Further, each of the dummy transistors LDTand UDT is not used for storing data. Each of the select transistors ST1and ST2 is used for selecting the string unit SU during variousoperations.

In each of the NAND strings NS, a drain of the select transistor ST1 isconnected to the associated bit line BL. The memory cell transistors MT5to MT15 are connected in series between a source of the selecttransistor ST1 and a drain of the dummy transistor UDT. A source of thedummy transistor UDT is connected to a drain of the dummy transistorLDT. The memory cell transistors MT0 to MT7 are connected in seriesbetween a source of the dummy transistor LDT and a drain of the selecttransistor ST2.

In the same block BLK, gates of the select transistors ST1 included inthe string units SU0 to SU3 are commonly connected to the select gatelines SGD0 to SGD3, respectively. Control gates of the memory celltransistors MT0 to MT15 are commonly connected respectively to wordlines WL0 to WL15. Control gates of the dummy transistors UDT and LDTare commonly connected respectively to dummy word lines UDWL and LDWL. Agate of the select transistor ST2 is commonly connected to a select gateline SGS.

Different column addresses are allocated to the bit lines BL0 to BLm.Each of the bit lines BL is commonly connected to the select transistorST1 of the corresponding NAND string NS between the blocks BLK. Each ofthe word lines WL0 to WL15 and the dummy word lines UDWL and LDWL isprovided for each of the blocks BLK. A source line SL is shared between,for example, the blocks BLK.

A group of the memory cell transistors MT connected to the common wordline WL in the single string unit SU is referred to as a cell unit CU,for example. For example, the storage capacity of the cell unit CUincluding the memory cell transistors MT each storing 1 bit data isdefined as “one page data”. The cell unit CU may have a storage capacityof two or more page data according to the number of bits of data storedin the memory cell transistor MT.

It should be noted that the configuration of the NAND-type flash memory10 described above is merely an example, and the present invention isnot limited thereto. For example, the number of the memory celltransistors MT and the select transistors ST1 and ST2 included in eachof the NAND strings NS may be designed in any number. The number of thestring units SU included in each of the blocks BLK may be designed inany number. The arrangement and number of transistors to be set as dummytransistors may be determined to arbitrary arrangement and number.

(Configuration of Memory Cell Array 11)

Hereinafter, an example of a configuration of the memory cell array 11included in the NAND-type flash memory 10 in the first embodiment willbe described. In the drawings referred to below, the X directioncorresponds to an extending direction of the word line WL, the Ydirection corresponds to an extending direction of the bit line BL, andthe Z direction corresponds to a vertical direction corresponding to asurface of a semiconductor substrate 40 on which the NAND-type flashmemory 10 is formed. In the cross-sectional views referred to below,constituent elements such as an insulating layer (interlayer insulatingfilm), wiring, and a contact are appropriately omitted in order to makethe figure easy to see. In the plan view, hatching is given asappropriate to make the figure easy to see. The hatching given to theplan view is not necessarily related to the material or characteristicsof the hatched component.

FIG. 5 is an example of a planar layout of the memory cell array 11included in the NAND-type flash memory 10 in the first embodiment, andextracts and illustrates the respective structures corresponding to thestring units SU0 and SU1. As shown in FIG. 5, a region where the memorycell array 11 is formed includes, for example, a plurality of slits SLT,the string units SU, and the bit lines BL.

Each of the slits SLT extends in the X direction and is arranged in theY direction. For example, the single string unit SU is disposed betweenthe slits SLT adjacent to each other in the Y direction.

Each of the string units SU includes a plurality of memory pillars MP.The memory pillars MP are arranged, for example, in a zigzag mannerspreading in the XY plane. Each of the memory pillars MP functions as,for example, the single NAND string NS.

The bit lines BL each extend in the Y direction and are arranged in theX direction. For example, each of the bit lines BL is disposed so as tooverlap with at least one of the memory pillars MP for each of thestring units SU. In this example, the two bit lines BL are arranged tooverlap each other in each of the memory pillars MP.

A contact CP is provided between one of the bit lines BL overlapping thememory pillar MP and the memory pillar MP. Each of the memory pillars MPis electrically connected to the corresponding bit line BL via thecontact CP.

FIG. 6 is a cross-sectional view taken along the line VI-VI of FIG. 5and shows an example of a cross-sectional structure of the memory cellarray 11 included in the NAND-type flash memory 10 in the firstembodiment. As shown in FIG. 6, for example, the semiconductor substrate40, conductor layers 41 to 48, the memory pillar MP, and the contact CPare included in the region where the memory cell array 11 is formed.

Specifically, the conductor layer 41 is provided above the semiconductorsubstrate 40 with an insulating layer interposed therebetween. Forexample, the conductor layer 41 is formed in a plate shape extendingalong the XY plane and used as the source line SL. Although not shown,circuits such as the sense amplifier module 19 are provided in a regionbetween the semiconductor substrate 40 and the conductor layer 41.

On the conductor layer 41, the slits SLT extending along an XZ plane arearranged in the Y direction. An insulator is embedded in the slit SLT.

On the conductor layer 41 and between the adjacent slits SLT, theconductor layer 42, the eight conductor layers 43, the conductor layer44, the conductor layer 45, the eight conductor layers 46, and theconductor layer 47 are provided in order from the lower layer. Amongthese conductor layers, the conductor layers adjacent to each other inthe Z direction are stacked via an interlayer insulating film. Each ofthe conductor layers 42 to 47 is formed in a plate shape along the XYplane.

The conductor layer 42 is used as the select gate line SGS. The eightconductor layers 43 are used respectively as the word lines WL0 to WL7in order from the lower layer. The conductor layers 44 and 45 are usedas the dummy word lines LDWL and UDWL, respectively. The eight conductorlayers 46 are used respectively as the word lines WL8 to WL15 in orderfrom the lower layer. The conductor layer 47 is used as the select gateline SGD.

The conductor layer 48 is provided above the conductor layer 47 with aninsulating layer interposed therebetween. For example, the conductorlayer 48 is formed in a line shape extending along the Y direction andis used as the bit line BL. That is, the conductor layers 48 arearranged along the X direction in a region not shown.

Each of the memory pillars MP is formed in a columnar shape extendingalong the Z direction and penetrates, for example, the conductor layers42 to 47. For example, an upper end of the memory pillar MP is includedin a layer between a layer provided with the conductor layer 47 and alayer provided with the conductor layer 48. A lower end of the memorypillar MP is in contact with the conductor layer 41.

Further, each of the memory pillars MP includes a plurality of columnarportions connected in the Z direction. Specifically, the structure ofthe memory pillar MP may be classified into a lower pillar LMP, an upperpillar UMP, and a joint JT.

The upper pillar UMP is provided above the lower pillar LMP. The lowerpillar LMP and the upper pillar UMP are bonded to each other, forexample, via the joint JT. For example, the outer diameter of the jointJT is larger than the outer diameter of a contact portion between thelower pillar LMP and the joint JT, and larger than the outer diameter ofa contact portion between the upper pillar UMP and the joint JT. Thedistance in the Z direction of a joint layer provided with the joint JT(the distance between the conductor layers 44 and 45) is wider than thedistance between the adjacent conductor layers 43 and is wider than thedistance between the adjacent conductor layers 46.

A detailed configuration inside the memory pillar MP will be describedbelow.

FIG. 7 is a cross-sectional view taken along the line VII-VII of FIG. 6and shows an example of a cross-sectional structure of the memory pillarMP and the word line WL in a layer including the conductor layer 43. Asshown in FIG. 7, the memory pillar MP includes, for example, asemiconductor member 50, a tunnel oxide film 51, an insulating film 52,and a block insulating film 53.

In the layer including the conductor layer 43, the semiconductor member50 is provided at a central portion of the memory pillar MP. The tunneloxide film 51 surrounds a side surface of the semiconductor member 50.The insulating film 52 surrounds a side surface of the tunnel oxide film51. The block insulating film 53 surrounds a side surface of theinsulating film 52. The conductor layer 43 surrounds a side surface ofthe block insulating film 53. In other words, the block insulating film53 is provided on an inner wall of a memory hole forming a memory pillarMH. The insulating film 52 is provided on an inner wall of the blockinsulating film 53. The tunnel oxide film 51 is provided on an innerwall of the insulating film 52. The semiconductor member 50 is providedon an inner wall of the tunnel oxide film 51.

The cross-sectional structure of the memory pillar MP in other portionsis the same as the cross-sectional structure described with reference toFIG. 7, for example, so that the description thereof is omitted. In thememory pillar MP, a material different from that of the semiconductormember 50 may be included in an inner wall of the semiconductor member50.

In the configuration of the memory pillar MP described above, forexample, a portion where the memory pillar MP intersects the conductorlayer 42 functions as the select transistor ST2. A portion where thememory pillar MP intersects the conductor layer 43 and a portion wherethe memory pillar MP intersects the conductor layer 46 each function asthe memory cell transistor MT.

A portion where the memory pillar MP intersects the conductor layer 44functions as the dummy transistor LDT. A portion where the memory pillarMP intersects the conductor layer 45 functions as the dummy transistorLDT. A portion where the memory pillar MP intersects the conductor layer46 functions as the select transistor ST1.

That is, the semiconductor member 50 functions as a channel of each ofthe memory cell transistor MT, the dummy transistors LDT and UDT, andthe select transistors ST1 and ST2. The insulating film 52 functions asa charge accumulation layer of the memory cell transistor MT. Each ofthe select transistor ST2, the memory cell transistors MT0 to MT7, andthe dummy transistor LDT is provided corresponding to the lower pillarLMP. Each of the select transistor ST1, the memory cell transistors MT8to MT15, and the dummy transistor UDT is provided corresponding to theupper pillar UMP.

Returning to FIG. 6, a columnar contact CP is provided on thesemiconductor member 50 in each of the memory pillars MP. The singleconductor layer 48, that is, the single bit line BL is in contact withan upper surface of the contact CP. The memory pillar MP and theconductor layer 48 may be electrically connected via two or morecontacts or may be electrically connected via other wiring.

In one example of the structure of the memory cell array 11 describedabove, the structure on the conductor layer 41 and between the adjacentslits SLT corresponds to the string unit SU. The present invention isnot limited thereto, and the structure of the memory cell array 11 maybe other structures.

For example, the number of the string units SU provided between theadjacent slits SLT may be designed in any number. The number andarrangement of the memory pillars MP shown in FIG. 5 are merelyexamples, and the memory pillars MP may be designed in any number andarrangement. The number of the bit lines BL overlapping with each of thememory pillars MP may be designed in any number.

The memory cell transistor MT, the dummy transistors UDT and LDT, andthe select transistors ST1 and ST2 included in each of the NAND stringNS may each be designed in any number. The number of the word lines WL,the dummy word lines UDWL and LDWL, and the select gate lines SGD andSGS may be changed based on the number of the memory cell transistorsMT, the dummy transistors UDT and LDT, and the select transistors ST1and ST2, respectively. The conductor layers 42 provided in a pluralityof layers may be allocated to the select gate line SGS. The conductorlayers 47 provided in a plurality of layers may be allocated to theselect gate line SGD.

The memory pillar MP may have a structure in which a pillar penetratingthe conductor layer 47 and a pillar penetrating the other conductorlayers 42 to 46 are connected in the Z direction. Each of the memorypillars MP may not have the joint JT. When each of the memory pillars MPdoes not have the joint JT, the upper pillar UMP and the lower pillarLMP in the memory pillar MP are directly connected without interpositionof the joint JT. A conductor layer used as a dummy word line may beincluded between the lowermost conductor layer 43 and the conductorlayer 42. Similarly, a conductor layer used as a dummy word line may beincluded between the uppermost conductor layer 46 and the conductorlayer 47.

[1-1-4] Circuit Configuration of Row Decoder Module 18

FIG. 8 shows an example of a circuit configuration of the row decodermodule 18 included in the NAND-type flash memory 10 in the firstembodiment and further shows wiring between the voltage generator 17 andthe memory cell array 11. As shown in FIG. 8, the row decoder module 18includes, for example, row decoders RD0 to RDn. The row decoders RD0 toRDn are associated with the blocks BLK0 to BLKn, respectively.

Hereinafter, a detailed circuit configuration of the row decoder RD willbe described, focusing on the row decoder RD0 corresponding to the blockBLK0. Since circuit configurations of the other row decoders RD aresimilar to the circuit configuration of the row decoder RD0, thedescription thereof will be omitted.

The row decoder RD includes, for example, a block decoder BD andtransistors TR0 to TR22. The block decoder BD decodes the block addressand applies a predetermined voltage to a transfer gate line TG based onthe decoding result. The transfer gate line TG is commonly connected togates of the transistors TR0 to TR22. Each of the transistors TR0 toTR22 is an n-channel MOS transistor with high withstand voltage.

The transistor TR is connected between a signal line wired from thevoltage generator 17 and a wire provided in the block BLK0.Specifically, a drain of the transistor TR0 is connected to a signalline SGSD. A source of the transistor TR0 is connected to the selectgate line SGS of the block BLK0.

Drains of the transistors TR1 to TR8 are connected to the signal linesCG0 to CG7, respectively. Sources of the transistors TR1 to TR8 areconnected to the respective one ends of the word lines WL0 to WL7corresponding to the block BLK0. A drain of the transistor TR9 isconnected to a signal line LCGD. A source of the transistor TR9 isconnected to the dummy word line LDWL. A drain of the transistor TR10 isconnected to a signal line UCGD. A source of the transistor TR10 isconnected to the dummy word line UDWL.

Drains of the transistors TR11 to TR18 are connected to the signal linesCG8 to CG15, respectively. Sources of the transistors TR11 to TR18 areconnected to the respective one ends of the word lines WL8 to WL15corresponding to the block BLK0. Drains of the transistors TR19 to TR22are connected to signal lines SGDD0 to SGDD3, respectively. Sources ofthe transistors TR19 to TR22 are connected to select gate lines SGD0 toSGD3, respectively.

With the above configuration, the row decoder module 18 can select theblock BLK executing various operations. Specifically, during variousoperations, the block decoder BD corresponding to the selected block BLKapplies a voltage of “H” level to the transfer gate line TG, and theblock decoder BD corresponding to the unselected block BLK applies avoltage of “L” level to the transfer gate line TG.

In this specification, the “H” level corresponds to the voltage at whichthe n-channel MOS transistor is turned on and the p-channel MOStransistor is turned off. The “L” level corresponds to the voltage atwhich the n-channel MOS transistor is turned off and the p-channel MOStransistor is turned on.

For example, when the block BLK0 is selected, the transistors TR0 toTR22 included in the row decoder RD0 are turned on, and the transistorsTR0 to TR22 included in the other row decoders RD are turned off. Inthis case, a current path is formed between various wires provided inthe block BLK0 and the corresponding signal line, and a current pathbetween various wires provided in another block BLK and thecorresponding signal line is shut off.

As a result, a voltage applied to each signal line by the voltagegenerator 17 is applied to various wires provided in the selected blockBLK0 via the row decoder RD0. The row decoder module 18 can operate inthe same manner even when another block BLK is selected.

It should be noted that the configuration of the row decoder module 18described above is merely an example, and the present invention is notlimited thereto. For example, the number of the transistors TR includedin the row decoder module 18 may be designed in number based on thenumber of wires provided in each of the blocks BLK.

[1-1-5] Circuit Configuration of Sense Amplifier Module 19

FIG. 9 shows an example of a circuit configuration of the senseamplifier module 19 included in the NAND-type flash memory 10 in thefirst embodiment. As shown in FIG. 9, the sense amplifier module 19includes, for example, sense amplifier units SAU0 to SAUm. The senseamplifier units SAU0 to SAUm are associated respectively with the bitlines BL0 to BLm.

Each of the sense amplifier units SAU includes, for example, a senseamplifier part SA and latch circuits SDL, ADL, BDL, and XDL. The senseamplifier part SA and the latch circuits SDL, ADL, BDL, and XDL areconnected so as to be capable of transmit and receive data to and fromeach other.

The sense amplifier part SA determines whether the read data is “0” or“1” based on the voltage of the corresponding bit line BL in the readoperation, for example. In other words, the sense amplifier part SAsenses the read data on the corresponding bit line BL and determinesdata to be stored in the selected memory cell.

Each of the latch circuits SDL, ADL, BDL, and XDL temporarily retainsread data, write data, and the like. The latch circuit XDL is connectedto an input/output circuit (not shown) and may be used for datainput/output between the sense amplifier unit SAU and the input/outputcircuit. The latch circuit XDL may be used as a cache memory of theNAND-type flash memory 10. For example, even if the latch circuits SDL,ADL, and BDL are being used, the NAND-type flash memory 10 can be heldin the ready state if the latch circuit XDL is not used.

FIG. 10 shows an example of a circuit configuration of the senseamplifier unit SAU included in the sense amplifier module 19 included inthe NAND-type flash memory 10 in the first embodiment. As shown in FIG.10, the sense amplifier part SA includes, for example, transistors 60 to68 and a capacitor 69. The latch circuit SDL includes, for example,transistors 70 and 71 and inverters 72 and 73.

For example, the transistor 60 is a p-channel MOS transistor. Each ofthe transistors 61 to 68, 70 and 71 is an n-channel MOS transistor.Transistor 63 is an n-channel MOS transistor with high withstandvoltage.

One end of the transistor 60 is connected to a power supply line. A gateof the transistor 60 is connected to a node INV (SDL) of the latchcircuit SDL. For example, a power supply voltage Vdd is applied to thepower supply line connected to one end of the transistor 60. One end ofthe transistor 61 is connected to the other end of the transistor 60.The other end of the transistor 61 is connected to a node COM. A controlsignal BLX is input to a gate of the transistor 61.

One end of the transistor 62 is connected to the node COM. A controlsignal BLC is input to a gate of the transistor 62. One end of thetransistor 63 is connected to the other end of the transistor 62. Theother end of the transistor 63 is connected to the corresponding bitline BL. A control signal BLS is input to a gate of the transistor 63.One end of the transistor 64 is connected to the node COM. The other endof the transistor 64 is connected to a node SRC. A gate of thetransistor 64 is connected to the node INV (SDL). For example, a groundvoltage Vss is applied to the node SRC. One end of the transistor 65 isconnected to the other end of the transistor 60. The other end of thetransistor 65 is connected to a node SEN. A control signal HLL is inputto a gate of the transistor 65.

One end of the transistor 66 is connected to the node SEN. The other endof the transistor 66 is connected to the node COM. A control signal XXLis input to a gate of the transistor 66. One end of the transistor 67 isgrounded. A gate of the transistor 67 is connected to the node SEN. Oneend of the transistor 68 is connected to the other end of the transistor67. The other end of the transistor 68 is connected to a bus LBUS. Acontrol signal STB is input to a gate of the transistor 68. One end ofthe capacitor 69 is connected to the node SEN. A clock CLK is input tothe other end of the capacitor 69.

In the latch circuit SDL, one end of each of the transistors 70 and 71is connected to the bus LBUS. The other ends of the transistors 70 and71 are connected to the node INV and a node LAT, respectively. Controlsignals STI and STL are input to the respective gates of the transistors70 and 71. An input node of the inverter 72 and an output node of theinverter 73 are connected to the node LAT. An output node of theinverter 72 and an input node of the inverter 73 are connected to thenode INV.

The circuit configuration of the latch circuits ADL, BDL and XDL is thesame as the circuit configuration of the latch circuit SDL, for example.On the other hand, in the latch circuit ADL, control signals ATI and ATLare input to the respective gates of the transistors 70 and 71. In eachof the latch circuits BDL and XDL, a control signal different from thatfor the latch circuit SDL is input to each of the transistors 70 and 71.The nodes INV and LAT of each of the latch circuits SDL, ADL, BDL, andXDL are each independently provided.

Each of the control signals BLX, BLC, BLS, HLL, XXL, STB, STI, STL, ATIand ATL described above is generated by the sequencer 15, for example.For example, the sequencer 15 can independently control the latchcircuits SDL, ADL, BDL, and XDL.

The timing at which the sense amplifier part SA determines data read outto the bit line BL is based on the timing at which the sequencer 15asserts the control signal STB. In the following description, theexpression “asserts the control signal STB” corresponds to an operationin which the sequencer 15 temporarily changes the control signal STBfrom “L” level to “H” level.

It should be noted that the configuration of the sense amplifier module19 described above is merely an example, and the present invention isnot limited thereto. For example, the number of latch circuits includedin the sense amplifier module 19 may be appropriately changed based onthe number of bits of data stored in the memory cell transistor MT.Depending on the circuit configuration of the sense amplifier module 19,the operation corresponding to the expression “asserts the controlsignal STB” may correspond to an operation in which the sequencer 15temporarily changes the control signal STB from “H” level to “L” level.

[1-1-6] Data Assignment

FIG. 11 shows an example of threshold distribution, read voltage, andverify voltage of the memory cell transistor MT in the memory system 1according to the first embodiment. In the threshold distribution shownin FIG. 11, the vertical axis corresponds to the number of the memorycell transistors MT, and the horizontal axis corresponds to a thresholdvoltage of the memory cell transistor MT. As shown in FIG. 11, in thememory system 1 according to the first embodiment, for example, eighttypes of threshold distributions may be formed by the threshold voltagesof the memory cell transistors MT included in the single cell unit CU.

In this specification, the eight types of threshold distributions (writestates) are referred to as “ER” state, “A” state, “B” state, “C” state,“D” state, “E” state, “F” state, and “G” state in order from the lowerthreshold voltage.

Read voltages to be used in the read operation are respectively setbetween adjacent threshold distributions. For example, a read voltage ARis set between the maximum threshold voltage at the “ER” state and theminimum threshold voltage at the “A” state. Likewise, a read voltage BRis set between the “A” state and the “B” state. A read voltage CR is setbetween the “B” state and the “C” state. A read voltage DR is setbetween the “C” state and the “D” state. A read voltage ER is setbetween the “D” state and the “E” state. A read voltage FR is setbetween the “E” state and the “F” state. A read voltage GR is setbetween the “F” state and the “G” state.

For example, in a case where the read voltage AR is applied to the gateof the memory cell transistor MT, the memory cell transistor MT isturned on when the threshold voltage is distributed at the “ER” state,and the memory cell transistor MT is turned off when the thresholdvoltage is distributed at the “A” state or more. Similarly, in a casewhere the read voltage BR is applied to the gate of the memory celltransistor MT, the memory cell transistor MT is turned on when thethreshold voltage is included at the “A” state or less, and the memorycell transistor MT is turned off when the threshold voltage is includedat the “B” state or more. Even when another read voltage is applied tothe gate of the memory cell transistor MT, the memory cell transistor MTis appropriately turned on or off according to the threshold voltage.

As a voltage higher than the highest threshold distribution, a read passvoltage Vread is set. Specifically, the read pass voltage Vread is setto be higher than the maximum threshold voltage at the “G” state, forexample. When the read pass voltage Vread is applied to the gate of thememory cell transistor MT, the memory cell transistor MT is retained inan on state regardless of stored data.

The verify voltages to be used in the write operation are respectivelyset between adjacent threshold distributions. Specifically, verifyvoltages AV, BV, CV, DV, EV, FV, and GV are set respectivelycorresponding to the “A” state, the “B” state, the “C” state, the “D”state, the “E” state, the “F” state, and the “G” state.

For example, the verify voltage AV is set between the maximum thresholdvoltage at the “ER” state and the minimum threshold voltage at the “A”state and near the “A” state. The verify voltage BV is set between themaximum threshold voltage at the “A” state and the minimum thresholdvoltage at the “B” state and near the “B” state. Similarly, other verifyvoltages are set near the corresponding write state. That is, the verifyvoltages AV, BV, CV, DV, EV, FV and GV are set to be higher than theread voltages AR, BR, CR, DR, ER, FR and GR, respectively.

In the write operation, when the memory system 1 detects that thethreshold voltage of the memory cell transistor MT storing certain datahas exceeded the verify voltage corresponding to this data, the memorysystem 1 completes a program of the memory cell transistor MT.

Different 3-bit data are assigned to the threshold distributions of theeight types of memory cell transistors MT described above. Below is anexample of data assignment for threshold distribution.

“ER” state: “111 (high order bit/middle order bit/low order bit)” data

“A” state: “110” data

“B” state: “100” data

“C” state: “000” data

“D” state: “010” data

“E” state: “011” data

“F” state: “001” data

“G” state: “101” data.

When such data assignment is applied, one page data (lower page data)including low order bits is determined by read processing using the readvoltages AR and ER. One page data (middle page data) including middleorder bits is determined by read processing using the read voltages BR,DR and FR. One page data (upper page data) including high order bits isdetermined by read processing using the read voltages CR and GR.

That is, the lower page data, the middle page data, and the upper pagedata are determined by the read processing using two types, three types,and two types of read voltages, respectively. Such data assignment isreferred to as “2-3-2 code”, for example. In this specification, thecase where the “2-3-2 code” is applied to data assignment to the memorycell transistor MT will be described as an example.

[1-2] Operation

Next, an operation of the memory system 1 according to the firstembodiment will be described. The memory system 1 according to the firstembodiment spontaneously executes the patrol operation during a periodin which operation based on an instruction from the host device 2 is notbeing executed. In the patrol operation, an optimum read voltage issearched using a tracking read or a shift read, and a correction valueis recorded on the LUT. Further, the patrol operation has a purpose ofdetecting defects to check whether or not data stored in the cell unitCU can be read out. Details of the tracking read, the shift read, andthe patrol operation will be sequentially described below.

[1-2-1] Tracking Read

An initial value of a read voltage is set such that the number of errorbits decreases when, for example, the memory cell transistors MTincluded in the cell unit CU form ideal threshold distribution. However,there is a possibility that the threshold voltage of the memory celltransistor MT changes after the write operation.

Thus, the memory system 1 according to the first embodiment executes thetracking read in order to detect valleys of the threshold distribution.The tracking read executed by the memory system 1 according to the firstembodiment is executed inside the NAND-type flash memory 10.

FIG. 12 shows an example of the read voltage used in tracking read inthe NAND-type flash memory 10 in the first embodiment and two adjacentthreshold distributions (“B” state and “C” state). In FIG. 12, the solidline corresponds to the threshold distribution immediately afterwriting, and the broken line corresponds to the threshold distributionafter the threshold voltage fluctuates. As shown in FIG. 12, even thoughthe threshold distribution of the memory cell transistor MT is ideallydistributed immediately after the write operation, a reduction due topassage of time or an increase due to read disturbance or the like mayoccur after the write operation.

Further, in the memory cell transistor MT in which a cycle of writingand erasing is repeated, the data retention characteristics maydeteriorate, and the threshold voltage may more significantly vary. Whensuch a variation in the threshold voltage occurs, the number of errorbits increases in read operation using a preset read voltage, and thereis a possibility that error correction becomes difficult.

On the other hand, the NAND-type flash memory 10 in the first embodimentcan search an optimum read voltage by executing tracking read. In thetracking read, read operation using a plurality of read voltages isexecuted, and a read voltage at which the number of error bits is thesmallest is searched.

For example, in the tracking read corresponding to the read voltage CR,read operations using respective tracking voltages CRt1, CRt2, CRt3,CRt4 and CRt5 are executed continuously. The read operations thusexecuted continuously using the tracking voltages are hereinafterreferred to as tracking.

The tracking voltages are set to arbitrary values, and a distance(step-up voltage) from an adjacent tracking voltage is set to besubstantially constant, for example. A relationship between thesevoltage values is CRt1<CRt2<CRt3<CRt4<CRt5. A relationship with theinitial value of the read voltage is CRt1<CR<CRt5.

In the tracking read corresponding to the other read voltages, thetracking voltage is set similarly to the tracking read corresponding tothe read voltage CR. The number of tracking voltages used in thetracking read may be different for each read voltage, and may be set toan arbitrary number. Similarly, the step-up voltage in the tracking readmay be different for each read voltage, and may be set to an arbitrarynumber.

When the read operations using the respective tracking voltages CRt1,CRt2, CRt3, CRt4 and CRt5 are executed in the tracking readcorresponding to the read voltage CR, the sequencer 15 detects a valleyportion between the threshold distribution at the “B” state and thethreshold distribution at the “C” state based on, for example, thenumber of ON cells of the memory cell transistor MT.

Then, the sequencer 15 determines the optimum read voltage based on thedetected valley portion of the threshold distribution. Thereafter, thesequencer 15 executes read operation using an estimated optimum readvoltage. In this read operation, for example, one of the trackingvoltages used for the tracking read is used as the optimum read voltage.

Hereinafter, a command sequence corresponding to the tracking read, avoltage applied to the selected word line WL, and operation of thecontrol signal STB will be described.

In the following description, the selected word line WL is referred toas a selected word line WLsel. It is assumed that a ready/busy signalRBn is “H” level (ready state) before the NAND-type flash memory 10starts operation, the voltage of the selected word line WLsel is Vss,and the control signal STB is “L” level.

In the operation described below, a voltage is applied to the selectedword line WLsel by the voltage generator 17 and the row decoder module18. Address information received by the NAND-type flash memory 10 isheld in the address register 13B. A command received by the NAND-typeflash memory 10 is held in the command register 13C.

FIG. 13 is a timing chart showing an example of tracking read in thememory system 1 according to the first embodiment and shows an exampleof tracking read corresponding to read operation of the upper page. Asshown in FIG. 13, when tracking read of the upper page is executed, thememory controller 20 transmits, for example, a command “xxh”, a command“03h”, a command “00h”, address information “ADD” of five cycles, and acommand “30h” to the NAND-type flash memory 10 in this order.

The command “xxh” is a command instructing the NAND-type flash memory 10to execute tracking read. The command “03h” is a command instructing theNAND-type flash memory 10 to perform the operation corresponding to theupper page. The command “00h” is a command instructing the NAND-typeflash memory 10 to execute read operation. The address information “ADD”of five cycles is address information corresponding to the cell unit CUto be operated and may include a block address, a column address, a pageaddress, and the like. The command “30h” is a command instructing theNAND-type flash memory 10 to start read operation based on a command, anaddress and the like received immediately before.

Upon receiving the command “30h”, the NAND-type flash memory 10transitions from the ready state to the busy state (RBn=“L” level), andthe sequencer 15 starts tracking read.

When the tracking read of the upper page starts, for example, thetracking voltages CRt1, CRt2, CRt3, CRt4 and CRt5 corresponding to theread voltage CR and tracking voltages GRt1, GRt2, GRt3, GRt4, and GRt5corresponding to the read voltage GR are sequentially applied to theselected word line WLsel.

The sequencer 15 asserts the control signal STB while each trackingvoltage is being applied to the selected word line WLsel. Then, thesequencer 15 estimates an optimum read voltage CRc corresponding to theread voltage CR based on the read results of the tracking voltages CRt1to CRt5 and estimates an optimum read voltage GRc corresponding to theread voltage GR based on the read results of the tracking voltages GRt1to GRt5. A correction value corresponding to the optimum read voltageobtained by the tracking read is held in the status register 13A, forexample.

Thereafter, the optimum read voltages CRc and GRc are sequentiallyapplied to the selected word line WLsel. The sequencer 15 asserts thecontrol signal STB while the optimum read voltages CRc and GRc are beingapplied to the selected word line WLsel.

The read result based on the optimum read voltage CRc is held in a latchcircuit ABL in each of the sense amplifier units SAU, for example.Thereafter, read data of the upper page is calculated based on the readresult based on the optimum read voltage GRc and the read result basedon the optimum read voltage CRc held in the latch circuit ABL. Thecalculation result is held in the latch circuit XDL in each of the senseamplifier units SAU, for example.

When the read data of the upper page is thus determined, the sequencer15 terminates the tracking read of the upper page and makes theNAND-type flash memory 10 transition from the busy state to the readystate.

The read result using the optimum read voltage obtained by the trackingread described above may be output to the memory controller 20 based onan instruction from the memory controller 20. The correction valuecorresponding to the optimum read voltage obtained by the tracking readis output to the memory controller 20 based on the instruction from thememory controller 20 and may be recorded in the LUT in the RAM 22, forexample.

The memory system 1 can also execute tracking read of each of the middleand lower pages in the same manner as the tracking read of the upperpage. In the tracking read of the lower page, for example, a command“01h” is used instead of the command “03h”. The command “01h” is acommand instructing the NAND-type flash memory 10 to perform theoperation corresponding to the lower page. In the tracking read of themiddle page, for example, a command “02h” is used instead of the command“03h”. The command “02h” is a command instructing the NAND-type flashmemory 10 to perform the operation corresponding to the middle page.

In the tracking read of each of the lower and middle pages, the trackingvoltage and the read voltage to be used are appropriately changed. Sincethe other operations in the tracking read of each of the lower andmiddle pages are the same as those in the tracking read of the upperpage, their explanations are omitted.

[1-2-2] Shift Read

In the memory system 1 according to the first embodiment, the shift readusing the tracking result (optimum read voltage) obtained by thetracking read is executed. The shift read is read operation in which theread voltage to be used is shifted from the initial value with respectto normal read operation. The shift amount of the read voltage in theshift read is set beforehand by setting change operation to be describedlater.

FIG. 14 is a timing chart showing an example of shift read in the memorysystem 1 according to the first embodiment and shows an example of shiftread corresponding to read operation of the upper page. As shown in FIG.14, when shift read of the upper page is executed, the memory controller20 transmits, for example, a command “yyh”, the command “03h”, thecommand “ooh”, the address information “ADD” of five cycles, and thecommand “30h” to the NAND-type flash memory 10 in this order.

The command “yyh” is a command instructing the NAND-type flash memory 10to execute shift read. Upon receiving the command “30h”, the NAND-typeflash memory 10 transitions from the ready state to the busy state(RBn=“L” level), and the sequencer 15 starts shift read.

When the shift read of the upper page starts, the optimum read voltagesCRc and GRc are applied sequentially. The sequencer 15 asserts thecontrol signal STB while the optimum read voltages CRc and GRc are beingapplied to the selected word line WLsel.

For example, the read result based on the optimum read voltage CRc isheld in the latch circuit ABL in each of the sense amplifier units SAU,for example. Thereafter, read data of the upper page is calculated basedon the read result based on the optimum read voltage GRc and the readresult based on the optimum read voltage CRc held in the latch circuitABL. The calculation result is held in the latch circuit XDL in each ofthe sense amplifier units SAU, for example.

When the read data of the upper page is thus determined, the sequencer15 terminates the shift read of the upper page and makes the NAND-typeflash memory 10 transition from the busy state to the ready state. Then,the read result using the optimum read voltage obtained by the shiftread is output to the memory controller 20 based on an instruction fromthe memory controller 20.

The memory system 1 can also execute shift read corresponding to each ofthe middle and lower pages in the same manner as the shift read of theupper page. In the shift read of the lower page, for example, thecommand “01h” is used instead of the command “03h”. In the shift read ofthe middle page, for example, the command “02h” is used instead of thecommand “03h”. In the shift read of each of the lower and middle pages,the optimum read voltage to be used is appropriately changed. Since theother operations in the shift read of each of the lower and middle pagesare the same as those in the shift read of the upper page, theirdescriptions are omitted.

[1-2-3] Patrol Operation

The memory system 1 according to the first embodiment spontaneouslyexecutes the patrol operation during a period in which operation basedon an instruction from the host device 2 is not being executed. That is,the memory system 1 according to the first embodiment executes thepatrol operation independently of the instruction from the host device 2during background operation.

The patrol operation contributes to reduction of read error in thememory system 1 and detection of the block BLK in which a defect hasoccurred. For example, the patrol operation is executed with respect toall the pages in all the blocks BLK for every predetermined repeatedperiod. In other words, all the pages in all the blocks BLK included ineach of the NAND-type flash memories 10 are subjected to at least onepatrol operation for each predetermined period of time.

In the following description, the predetermined period of time duringwhich the patrol operation is thus executed is referred to as a patrolperiod. The patrol period is set based on the time measured by the timer28. The time corresponding to one cycle of the patrol period is, forexample, one day. The present invention is not limited thereto, and thetime corresponding to one cycle may be set to an arbitrary length. Thememory system 1 according to the first embodiment executes historylearning and defect detection, for example, in the patrol operation.

The history learning is an operation of confirming an optimum readvoltage on a page targeted for the patrol operation. In the historylearning, for example, tracking read is executed. A correction value(tracking result) of the read voltage obtained by the tracking read isappropriately transferred to the memory controller 20.

The defect detection is an operation of confirming whether or not it ispossible to read data corresponding to the page targeted for the patroloperation. In the defect detection, for example, shift read using atracking result is executed. The data read by shift read is transferredto the memory controller 20, and error correction processing is executedby the ECC circuit 24.

In the defect detection, if error correction of read data is possible,it is judged that the relevant page can read data. On the other hand, ifthe error correction of read data is impossible, it is judged that therelevant page is a bad page. For example, if a bad page is included inthe block BLK in which the patrol operation is executed, data written inthis block BLK is saved in another block BLK.

As described above, the purpose of the patrol operation is to create thecorrection value of the read voltage by the history learning and toconfirm the presence/absence of a physical fault by the defectdetection. The history learning does not necessarily have to be executedfor all the word lines WL, and it is sufficient that the historylearning is executed for an arbitrarily set representative word line WL.On the other hand, the defect detection is aimed at detecting a physicaldefect such as a short circuit between the word lines WL adjacent toeach other, and therefore it is necessary to execute the defectdetection for all the word lines WL.

(Command Sequence in Patrol Operation)

FIG. 15 shows an example of a command sequence in the patrol operationof the memory system 1 according to the first embodiment. As shown inFIG. 15, for example, when starting the patrol operation, the memorysystem 1 executes the history learning first. In the history learning,the memory system 1 first executes a setting change operation of aparameter relating to tracking read. This “parameter” is data referredto by the sequencer 15 in various operations, and is held in theregister set 13, for example.

In the setting change operation, the memory controller 20 transmits, forexample, a command “EFh”, an address “zzh”, and data “D1”, “D2”, “D3”and “D4” to the NAND-type flash memory 10 in this order.

The command “EFh” is a command instructing the NAND-type flash memory 10to execute the setting change operation. The address “zzh” includes anaddress designating a parameter to be subjected to the setting changeoperation. The data “D1” to “D4” include parameters to be changed by thesetting change operation.

Upon receiving the data “D4”, the NAND-type flash memory 10 transitionsfrom the ready state to the busy state. Then, the sequencer 15 rewritesthe parameters as appropriate based on the received address “zzh” andthe data “D1” to “D4”. When the rewriting of the parameters iscompleted, the NAND-type flash memory 10 transitions from the busy stateto the ready state.

In this example, the memory system 1 executes a setting change operationof setting a start voltage of tracking read and a setting changeoperation of setting a step-up voltage in the tracking read beforeexecuting the tracking read. The present invention is not limitedthereto, and the setting change operation related to the tracking readmay be appropriately omitted.

When the setting change operation of the parameter relating to thetracking read is completed, the memory system 1 executes the trackingread. In the tracking read, the memory controller 20 transmits, forexample, the command “xxh”, the command “01h”, the address information“ADD” of five cycles, and the command “30h” to the NAND-type flashmemory 10 in this order.

Upon receiving the command “30h”, the NAND-type flash memory 10transitions from the ready state to the busy state and executes thetracking read as described with reference to FIG. 13. When the trackingread is completed, the NAND-type flash memory 10 transitions from thebusy state to the ready state. The illustrated time tTR corresponds toprocessing time of the tracking read.

When execution of the tracking read is completed, the memory system 1executes output of the tracking result. In the output of the trackingresult, the memory controller 20 transmits, for example, a command “B0h”and the command “00h” to the NAND-type flash memory 10. The command“B0h” is a command instructing the NAND-type flash memory 10 to outputthe tracking result.

Upon receiving the command “00h”, the NAND-type flash memory 10 outputsthe tracking result held in the status register 13A to the memorycontroller 20 (“STSout”), for example.

The tracking result received by the memory controller 20 is recorded inthe LUT in the RAM 22, for example. The tracking result may be held inthe DRAM 30, and it is sufficient that the tracking result is held in aregion other than the NAND package PKG in the memory system 1.

The operation described above corresponds to the history learning in thepatrol operation. When the output of the tracking result is completed,the memory system 1 enters a state capable of executing operation basedon an instruction from the host device 2 (“interruptible”).

At this time, if the memory system 1 has not received the instructionfrom the host device 2, the memory system 1 continues to execute defectdetection (shift read). On the other hand, when the memory system 1 hasreceived the instruction from the host device 2, the memory system 1executes an operation based on the instruction from the host device 2and then executes the defect detection.

When the operation based on the instruction from the host device 2 isexecuted, the read result held in the latch circuit XDL in the senseamplifier unit SAU is discarded. On the other hand, the correction valueof the read voltage obtained by the tracking read is maintained, forexample, in the status register 13A.

In the defect detection, the memory system 1 first executes a settingchange operation of a parameter relating to shift read. Since a commandsequence in the setting change operation is the same as the settingchange operation of the parameter relating to the tracking read, thedescription is omitted.

When this setting change operation is executed, parameters in theNAND-type flash memory 10 are appropriately rewritten based on thetracking result held in the RAM 22 in the memory controller 20, forexample. As a result, in the NAND-type flash memory 10, the read voltageduring execution of the shift read is set.

When the setting change operation of the parameter relating to the shiftread is completed, the memory system 1 executes the shift read. In theshift read, the memory controller 20 transmits, for example, the command“yyh”, the command “01h”, the address information “ADD” of five cycles,and the command “30h” to the NAND-type flash memory 10 in this order.

Upon receiving the command “30h”, the NAND-type flash memory 10transitions from the ready state to the busy state and executes theshift read as described with reference to FIG. 14. When the shift readis completed, the NAND-type flash memory 10 transitions from the busystate to the ready state. The illustrated time tSR corresponds to theprocessing time of the shift read. The time tSR is shorter than the timetTR.

When execution of the shift read is completed, the memory system 1executes output of read data. In the first embodiment, the “read data”indicates the read result obtained by the shift read and based on theoptimum read voltage.

In the output of the read data, the memory controller 20 transmits, forexample, a command “05h”, the address information “ADD” of five cycles,and a command “E0h” to the NAND-type flash memory 10 in this order. Thecommands “05h” and “E0h” are commands instructing the NAND-type flashmemory 10 to output the read data.

Upon receiving the command “E0h”, the NAND-type flash memory 10 outputsthe read data held in the latch circuit XDL in the sense amplifier unitSAU to the memory controller 20 (“Dout”), for example.

When the memory controller 20 receives the read data, the errorcorrection processing is executed by the ECC circuit 24. Then, based onthe result of the error correction processing, it is judged whether ornot a page on which the patrol operation is executed can be read out.

The operation described above corresponds to the defect detection in thepatrol operation. The memory system 1 according to the first embodimentprogresses the patrol operation on a page basis by executing the historylearning and the defect detection as described above.

In the memory system 1 according to the first embodiment, in theNAND-type flash memory 10 using the common bus line Ch, a processingunit (processing unit of a command) of patrol operation to be processedmay be set. Hereinafter, an example of operation in a case where theprocessing unit of the patrol operation is different in the memorysystem 1 according to the first embodiment will be exemplified.

(Case Where Processing Unit of Patrol Operation is 1 Page/1 BNK)

FIG. 16 is an example of processing order of the patrol operation in thememory system 1 according to the first embodiment and exemplifies a casewhere the processing unit of the patrol operation processed in each ofthe NAND-type flash memories 10 is 1 page/1 BNK. In this specification,the fact that “the processing unit of the patrol operation is 1 page/1BNK” indicates that the patrol operation is not executed in parallel inthe NAND-type flash memory 10 using the common bus line Ch, for example.Note that, the patrol operation may be executed in parallel in theNAND-type flash memory 10 using the different channel controllers CC.

As shown in FIG. 16, in this example, the patrol operation for a blockBLKx (x is an integer equal to or larger than 0) may be classified into,for example, a first period, a second period, and a third period. Thenumbers shown in each period correspond to an execution order of thepatrol operation. When the same numbers are indicated in the historylearning and the defect detection in the same period, this indicatesthat these operations are executed continuously, for example. “L”, “M”,and “U” shown in FIG. 16 correspond to the lower page, the middle page,and the upper page, respectively.

In each of the first period, the second period, and the third period,history learning and defect detection in which different pages areselected are executed in a set of the selected string unit SU and wordline WL.

Specifically, in the first period, the string unit SU0 and the word lineWL0 are first selected, and the history learning and defect detection ofthe lower page are executed (the first period, “1”). Next, the stringunit SU1 and the word line WL0 are selected, and the history learningand defect detection of the middle page are executed (the first period,“2”). Next, the string unit SU2 and the word line WL0 are selected, andthe history learning and defect detection of the upper page are executed(the first period, “3”). Next, the string unit SU3 and the word line WL0are selected, and the history learning and defect detection of the lowerpage are executed (the first period, “4”).

Next, the history learning and defect detection in which the word lineWL1 is selected are executed. At this time, the string unit SU and thepage are selected in the same order as in the case where the word lineWL0 is selected (the first period, “5” to “8”).

In the first period, when the history learning and defect detection inwhich the word line WL1 is selected are executed, for example, thehistory learning may be omitted in the subsequent patrol operation. Thatis, in the patrol operation in which the word line WL2 is selected, forexample, only the defect detection is executed (the first period, “9” to“12”). In the patrol operation in which the history learning is omitted,for example, the defect detection is executed by using the result of thehistory learning in another word line WL executed in the same block BLK.In the first period, the patrol operation in which another word line WLis selected is executed in the same order as the patrol operation inwhich the word line WL2 is selected, for example, so that thedescription will be omitted.

In the second period, the string unit SU0 and the word line WL0 arefirst selected, and the history learning and defect detection of themiddle page are executed (the second period, “1”). Next, the string unitSU1 and the word line WL0 are selected, and the history learning anddefect detection of the upper page are executed (the second period,“2”). Next, the string unit SU2 and the word line WL0 are selected, andthe history learning and defect detection of the lower page are executed(the second period, “3”). Next, the string unit SU3 and the word lineWL0 are selected, and the history learning and defect detection of themiddle page are executed (the second period, “4”).

Next, the history learning and defect detection in which the word lineWL1 is selected are executed. At this time, the string unit SU and thepage are selected in the same order as in the case where the word lineWL0 is selected (the second period, “5” to “8”).

In the second period, when the history learning and defect detection inwhich the word line WL1 is selected are executed, for example, thehistory learning may be omitted in the subsequent patrol operation as inthe first period (the second period, “9” to “12”). In the second period,the patrol operation in which another word line WL is selected isexecuted in the same order as the patrol operation in which the wordline WL2 is selected, for example, so that the description will beomitted.

In the third period, the string unit SU0 and the word line WL0 are firstselected, and the history learning and defect detection of the upperpage are executed (the third period, “1”). Next, the string unit SU1 andthe word line WL0 are selected, and the history learning and defectdetection of the lower page are executed (the third period, “2”). Next,the string unit SU2 and the word line WL0 are selected, and the historylearning and defect detection of the middle page are executed (the thirdperiod, “3”). Next, the string unit SU3 and the word line WL0 areselected, and the history learning and defect detection of the upperpage are executed (the third period, “4”).

Next, the history learning and defect detection in which the word lineWL1 is selected are executed. At this time, the string unit SU and thepage are selected in the same order as in the case where the word lineWL0 is selected (the third period, “5” to “8”).

In the third period, when the history learning and defect detection inwhich the word line WL1 is selected are executed, the history learningmay be omitted in the subsequent patrol operation as in the first period(the third period, “9” to “12”). In the third period, the patroloperation in which another word line WL is selected is executed in thesame order as the patrol operation in which the word line WL2 isselected, for example, so that the description will be omitted.

The patrol operation in the first to third periods is executed asdescribed above, whereby, in the memory system 1 according to the firstembodiment, the patrol operation corresponding to three pages in all theword lines WL in the block BLKx can be executed.

In the first to third periods, the order in which pages are selecteddiffers in the patrol operation in which each of the word lines WL isselected. In this example, at least one patrol operation is executed inall regions (all combinations of the word line WL and the string unit SUin the relevant block BLK) in each of the first to third periods.

There will be hereinafter described an operation timing of the patroloperation in a case where the patrol operations of the lower page, themiddle page, and the upper page are sequentially executed in each of theNAND-type flash memories 10 connected to the common bus line Ch.

FIG. 17 shows an example of the operation timing of the patrol operationin the case where the processing unit of the patrol operation is 1page/1 BNK in the first embodiment.

In a timing chart referred to in the following description, “BNK0” and“BNK1” indicate operation states in the NAND-type flash memory 10corresponding to the banks BNK0 and BNK1, respectively. “Ch0” indicatesa signal transmitted/received via the bus line Ch0. “TR(L)”, “TR(M)”,and “TR(U)” indicate processing periods of the tracking readcorresponding to the lower page, the middle page, and the upper page,respectively. “SR(L)”, “SR(M)”, and “SR(U)” indicate processing periodsof the shift read corresponding to the lower page, the middle page, andthe upper page, respectively.

In the following description of the timing chart, in order to simplifythe description, the NAND-type flash memory 10 corresponding to the bankBNK0 is referred to as “bank BNK0”, and the NAND-type flash memory 10corresponding to the bank BNK1 is referred to as “bank BNK1”. Further,it is assumed that the bus line Ch0 is used in the operation of thememory controller 20 transmitting a command to each of the banks BNK andthe operation of each of the banks BNK transmitting data or status tothe memory controller 20.

As shown in FIG. 17, the memory system 1 alternately executes operationcorresponding to the bank BNK0 and operation corresponding to the bankBNK1.

Specifically, first, the memory controller 20 transmits the command CMDcorresponding to the tracking read of the lower page to the bank BNK0.Then, the bank BNK0 executes the tracking read of the lower page andthen transmits the tracking result STSout to the memory controller 20(history learning of the lower page in BNK0).

Next, the memory controller 20 transmits the command CMD correspondingto the tracking read of the lower page to the bank BNK1. Then, the bankBNK1 executes the tracking read of the lower page and then transmits thetracking result STSout to the memory controller 20 (history learning ofthe lower page in BNK1).

Next, the memory controller 20 transmits the command CMD correspondingto the shift read of the lower page to the bank BNK0. Then, the bankBNK0 executes the shift read of the lower page and then transmits theread data Dout to the memory controller 20 (defect detection of thelower page in BNK0).

Next, the memory controller 20 transmits the command CMD correspondingto the shift read of the lower page to the bank BNK1. Then, the bankBNK0 executes the shift read of the lower page and then transmits theread data Dout to the memory controller 20 (defect detection of thelower page in BNK1).

In the first embodiment, when the processing unit of the patroloperation is 1 page/1 BNK, the memory controller 20 can interrupt readoperation (hereinafter referred to as host read) based on an instructionfrom the host device 2 at the end of the shift read and at the end ofthe tracking read.

Subsequently, the patrol operation corresponding to the middle page andthe patrol operation corresponding to the upper page are sequentiallyexecuted. Since the operation timings of the patrol operationscorresponding to the middle page and the upper page are the same as theoperation timing of the patrol operation corresponding to the lowerpage, the description thereof will be omitted.

(Case where Processing Unit of Patrol Operation is 1 WL/2 BNK)

FIG. 18 is an example of processing order of the patrol operation in thememory system 1 according to the first embodiment and exemplifies a casewhere the processing unit of the patrol operation processed in each ofthe NAND-type flash memories 10 is 1 WL/2 BNK. In this specification,the fact that “the processing unit of the patrol operation is 1 WL/2BNK” indicates that the patrol operation is executed in parallel in theNAND-type flash memory 10 using the common bus line Ch.

The numbers shown in FIG. 18 indicate an order in which a set of thepatrol operations of the lower page, the middle page, and the upper pageto be executed continuously is executed. When the same numbers areindicated in the history learning and the defect detection, thisindicates that these operations are executed continuously, for example.

As shown in FIG. 18, in this example, in the patrol operation for theblock BLKx, the period is not classified as in the example describedwith reference to FIG. 16, for example. In this example, the word lineWL0 is first selected, and the history learning and defect detection inwhich the string units SU0 to SU3 are sequentially selected are executed(“1” to “4”). Next, the word line WL1 is selected, and the historylearning and defect detection in which the string units SU0 to SU3 aresequentially selected are executed (“5” to “8”).

When the history learning and defect detection in which the word lineWL1 is selected are executed, for example, the history learning may beomitted in the subsequent patrol operation. That is, in the patroloperation in which the word line WL2 is selected, for example, only thedefect detection is executed (“9” to “12”). In the patrol operation inwhich the history learning is omitted, for example, the defect detectionis executed by using the result of the history learning executed in thesame block BLK. In this example, the patrol operation in which anotherword line WL is selected is executed in the same order as the patroloperation in which the word line WL2 is selected, for example, so thatthe description will be omitted.

The patrol operation is executed as described above, whereby, in thememory system 1 according to the first embodiment, the patrol operationcorresponding to three pages in all the word lines WL in the block BLKxcan be executed.

There will be hereinafter described an operation timing of the patroloperation in a case where the patrol operations of the lower page, themiddle page, and the upper page are sequentially executed in each of theNAND-type flash memories 10 connected to the common bus line Ch.

FIG. 19 shows an example of the operation timing of the patrol operationin the case where the processing unit of the patrol operation is 1 WL/2BNK in the first embodiment. As shown in FIG. 19, the memory system 1executes the operation corresponding to the bank BNK0 and the operationcorresponding to the bank BNK1 in parallel, as appropriate.

Specifically, first, the memory controller 20 sequentially transmits thecommand CMD corresponding to the tracking read of the lower page to thebanks BNK0 and BNK1. Upon receiving the command CMD corresponding to thetracking read of the lower page, each of the banks BNK0 and BNK1 startsthe tracking read of the lower page.

At this time, the tracking read in the bank BNK0 and the tracking readin the bank BNK1 can be executed in parallel. The tracking read of thelower page is completed in the order of the bank BNK0 and the bank BNK1.

When the tracking read of the lower page is completed, each of the banksBNK0 and BNK1 sequentially transmit the tracking result STSout to thememory controller 20 (history learning of the lower page in each of BNK0and BNK1).

Next, the memory system 1 sequentially executes the history learning ofthe middle page and the history learning of the upper page for each ofthe banks BNK0 and BNK1. Since these operation timings are the same asthose of the history learning of the lower page, the description will beomitted.

In the first embodiment, when the processing unit of the patroloperation is 1 WL/2 BNK, since the history learning and the defectdetection can be separated, the memory controller 20 can interrupt thehost read at this timing.

Upon completion of the history learning of the lower page, the middlepage, and the upper page, the memory system sequentially transmits thecommand CMD corresponding to the shift read of the lower page to thebanks BNK0 and BNK1.

Upon receiving the command CMD corresponding to the shift read of thelower page, each of the banks BNK0 and BNK1 starts the shift read of thelower page. At this time, the shift read in the bank BNK0 and the shiftread in the bank BNK1 can be executed in parallel.

When the shift read of the lower page in the bank BNK0 is completed, thememory controller 20 causes the bank BNK0 to output the read data Doutof the lower page (defect detection of the lower page in BNK0). Forexample, while the bank BNK0 is outputting the read data Dout, the shiftread of the lower page in the bank BNK1 is completed.

Upon receiving the read data Dout of the lower page from the bank BNK0,the memory controller 20 transmits the command CMD corresponding to theshift read of the middle page to the bank BNK0, and the bank BNK0 startsthe shift read of the middle page.

While the bank BNK0 is executing the shift read of the middle page, thememory controller 20 causes the bank BNK1 to output the read data Doutof the lower page (defect detection of the lower page in BNK1).

For example, the output of the read data Dout of the lower page in thebank BNK1 is completed while the bank BNK0 is executing the shift readof the middle page. In other words, the shift read of the middle page inthe bank BNK0 and the output of the read data Dout of the lower page inthe bank BNK1 can be executed in parallel;

Upon receiving the read data Dout of the lower page from the bank BNK1,the memory controller 20 transmits the command CMD corresponding to theshift read of the middle page to the bank BNK1, and the bank BNK1 startsthe shift read of the middle page.

For example, while the bank BNK1 is executing the shift read of themiddle page, the shift read of the middle page in the bank BNK0 iscompleted. In other words, the shift read of the middle page in the bankBNK1 and the output of the read data Dout of the middle page in the bankBNK0 can be executed in parallel.

When the shift read of the middle page in the bank BNK0 is completed,the memory controller 20 causes the bank BNK0 to output the read dataDout of the middle page (defect detection of the middle page in BNK0).Subsequently, the memory controller 20 transmits the command CMDcorresponding to the shift read of the upper page to the bank BNK0, andthe bank BNK0 starts the shift read of the upper page.

Thereafter, similarly, the shift read in the bank BNK0 and output ofread data in the bank BNK1 can be executed in parallel as appropriate,and the shift read in the bank BNK1 and output of read data in the bankBNK0 can be executed in parallel as appropriate.

In the patrol operation described above, the case where the historylearning and the defect detection are executed continuously isexemplified, but the present invention is not limited thereto. Forexample, in the patrol operation in the first embodiment, the historylearning and the defect detection may be separately executed. In thiscase, it is sufficient that the memory system 1 executes the defectdetection at least after executing the history learning for the pagetargeted for the patrol operation.

In the patrol operation described above, the case where the historylearning for the word line WL2 and the subsequent word lines is omittedis exemplified, but the present invention is not limited thereto. In thepatrol operation in the first embodiment, the history learning may beexecuted for all the word lines WL, and the word line WL from which thehistory learning is omitted may be arbitrarily selected.

[1-3] Effects of First Embodiment

According to the memory system 1 according to the first embodimentdescribed above, reliability of data to be stored can be guaranteed, andlatency can be improved. Hereinafter, effects of the memory system 1according to the first embodiment will be described in detail.

When error correction becomes impossible in read operation (hereinafterreferred to as host read) according to an instruction from the hostdevice 2, for example, an SSD (Solid State Drive) executes retry read.Restoration techniques such as Vth tracking can be applied to the retryread.

The Vth tracking is an operation of transmitting the read result basedon the tracking voltage as described with reference to FIG. 13 to thememory controller 20 and estimating a highly accurate correction valueof a read voltage. Thus, the processing time of the retry read tends tobe long, and when the retry read occurs, it becomes difficult to satisfya latency performance desired by a customer.

On the other hand, in the SSD used in a data center or the like, thepatrol operation is executed during the background operation. In thepatrol operation, an optimum read voltage of the memory cell is searchedby Vth tracking or shift read, and a correction value on the LUT isupdated.

Then, in the subsequent read operation, the SSD executes read operationusing the correction value updated on the LUT. That is, the higher theaccuracy of the correction value created by the patrol operation, themore it is possible to suppress the occurrence of the retry read in theSSD.

However, while the Vth tracking can create the correction value withhigh accuracy, there is a concern that data exchange between the memorycontroller 20 and the NAND-type flash memory 10 increases and a bus isoccupied. As a result, it becomes difficult to increase the number ofparallel processings between the banks BNK, so that the latencyincreases.

In a large capacity SSD, the number of chips allocated to a channeltends to increase, and a method of controlling the patrol operation inthe background operation becomes important. In particular, in order toguarantee the reliability of the SSD, it is necessary to create a highlyaccurate correction value of a read voltage while suppressing anincrease in latency due to the patrol operation.

Thus, in the memory system 1 according to the first embodiment, in thepatrol operation, tracking read creating the correction value of theread voltage in the NAND-type flash memory 10 is executed. The trackingread in the first embodiment has an advantage that data output as in theVth tracking is unnecessary and the bus is not occupied.

Consequently, in the memory system 1 according to the first embodiment,it is possible to increase the number of parallel processings of thepatrol operation between the banks BNK, and to increase processing speedof the patrol operation in the entire memory system 1. In the memorysystem 1 according to the first embodiment, a time for executing thepatrol operation is shortened.

As a result, in the memory system 1 according to the first embodiment,it is possible to suppress the occurrence of the retry read and toreduce probability of collision between the host read and the patroloperation. That is, the memory system 1 according to the firstembodiment can guarantee the reliability of data and improve thelatency.

In the above description, the advantage of executing the patroloperation in parallel has been described; however, the processing unitof optimum patrol operation changes according to the processing unit ofthe command. This point will be explained in the item of “Effects ofSecond Embodiment” to be described later.

[1-4] Variation of First Embodiment

In the first embodiment, the case where the processing unit is the samefor each of the history learning (tracking read) and the defectdetection (shift read) is exemplified, but the present invention is notlimited thereto. For example, the processing unit in the historylearning and the processing unit in the defect detection may bedifferent.

Hereinafter, an example of the operation timing of the patrol operationin the memory system 1 according to the variation of the firstembodiment will be described. In the variation of the first embodiment,the history learning is executed in parallel, and the defect detectionis executed on a page basis.

FIG. 20 shows an example of a timing chart of the patrol operation inthe case where the processing unit of the history learning is 1 page/2BNK and the processing unit of the defect detection is 1 page/1 BNK inthe variation of the first embodiment. As shown in FIG. 20, the memorysystem 1 executes operations corresponding to the banks BNK0 and BNK1 inparallel as appropriate in the history learning and sequentiallyexecutes operations corresponding to the banks BNK0 and BNK1 in thedefect detection.

Specifically, first, the memory controller 20 causes each of the banksBNK0 and BNK1 to execute the tracking read of the lower page similarlyto the operation described with reference to FIG. 19. When the trackingread of the lower page is completed, each of the banks BNK0 and BNK1sequentially transmit the tracking result STSout to the memorycontroller 20 (history learning of the lower page in each of BNK0 andBNK1).

Next, similarly to the operation described with reference to FIG. 17,the memory controller 20 causes the bank BNK0 to execute the shift readof the lower page and outputs the read data Dout of the lower page(defect detection of the lower page in BNK0).

Next, similarly to the operation described with reference to FIG. 17,the memory controller 20 causes the bank BNK1 to execute the shift readof the lower page and outputs the read data Dout of the lower page(defect detection of the lower page in BNK1).

Thereafter, similarly, the memory controller 20 causes each of the banksBNK0 and BNK1 to execute the tracking read of the middle page inparallel, causes each of the banks BNK0 and BNK1 to sequentially executethe shift read of the middle page, causes each of the banks BNK0 andBNK1 to execute the tracking read of the upper page in parallel, andcauses each of the banks BNK0 and BNK1 to sequentially execute the shiftread of the upper page.

In the variation of the first embodiment, the memory controller 20 caninterrupt the host read at the end of 2BNK operation of the historylearning and at the end of 1BNK operation of the defect detection,respectively.

By executing the patrol operation as described above, the memory system1 according to the variation of the first embodiment can shorten theprocessing time of the history learning in the patrol operation. Thatis, the memory system 1 according to the variation of the firstembodiment can speed up the patrol operation and improve the latency.

[2] Second Embodiment

A memory system 1 according to a second embodiment has the sameconfiguration as the memory system 1 according to the first embodiment,for example. The memory system 1 according to the second embodimentexecutes defect detection using a read result obtained by tracking readin patrol operation. Hereinafter, points of the memory system 1according to the second embodiment different from the first embodimentwill be described.

[2-1] Patrol Operation

(Command Sequence in Patrol Operation)

FIG. 21 shows an example of a command sequence in the patrol operationof the memory system 1 according to the first embodiment. As shown inFIG. 21, for example, when starting the patrol operation, the memorysystem 1 executes history learning first, as in the first embodiment.

Specifically, the memory system 1 first appropriately executes a settingchange operation corresponding to setting a start voltage of trackingread and a setting change operation corresponding to setting a step-upvoltage in the tracking read.

When the setting change operation of the parameter relating to thetracking read is completed, the memory system 1 executes the trackingread. Since the command sequence in each of the setting change operationand the tracking read is the same as that in the first embodiment, thedescription will be omitted.

Upon receiving the command “30h”, the NAND-type flash memory 10transitions from the ready state to the busy state and executes thetracking read as described with reference to FIG. 13. When the trackingread is completed, the NAND-type flash memory 10 transitions from thebusy state to the ready state.

When execution of the tracking read is completed, the memory system 1executes output of read data. In the first embodiment, the “read data”indicates the read result obtained by the tracking read and based on anoptimum read voltage.

The command sequence in the output of the read data is the same as thatin the first embodiment. Upon receiving the command “E0h”, the NAND-typeflash memory 10 outputs the read data held in the latch circuit XDL inthe sense amplifier unit SAU to the memory controller 20 (“Dout”), forexample.

When the memory controller 20 receives the read data, the errorcorrection processing is executed by the ECC circuit 24. Then, based onthe result of the error correction processing, it is judged whether ornot a page on which the patrol operation is executed can be read out.

As described above, the memory system 1 according to the secondembodiment executes the defect detection using the read result in thetracking read in the patrol operation.

(Case Where Processing Unit of Patrol Operation is 1 Page/1 BNK)

A processing order of the patrol operation in a case where a processingunit of the patrol operation is 1 page/1 BNK is the same in the secondembodiment and the first embodiment, for example.

Hereinafter, an example of an operation timing of the patrol operationin the case where the processing unit of the patrol operation is 1page/1 BNK in the memory system 1 according to the second embodimentwill be described.

FIG. 22 shows an example of a timing chart of the patrol operation inthe case where the processing unit of the patrol operation is 1 page/1BNK in the second embodiment. As shown in FIG. 22, the memory system 1alternately executes operation corresponding to a bank BNK0 andoperation corresponding to a bank BNK1.

Specifically, first, the memory controller 20 transmits the command CMDcorresponding to the tracking read of the lower page to the bank BNK0.Then, the bank BNK0 executes the tracking read of the lower page andthen transmits the read data Dout of the lower page to the memorycontroller 20 (patrol operation of the lower page in BNK0).

Next, the memory controller 20 transmits the command CMD correspondingto the tracking read of the lower page to the bank BNK1. Then, the bankBNK1 executes the tracking read of the lower page and then transmits theread data Dout of the lower page to the memory controller 20 (patroloperation of the lower page in BNK1).

In the second embodiment, when the processing unit of the patroloperation is 1 page/1 BNK, the memory controller 20 can interrupt hostread after completion of a set of the tracking read and the output ofthe read data executed every 1 BNK.

Subsequently, the patrol operation corresponding to the middle page andthe patrol operation corresponding to the upper page are sequentiallyexecuted. Since the operation timings of the patrol operationscorresponding to the middle page and the upper page are the same as theoperation timing of the patrol operation corresponding to the lowerpage, the description thereof will be omitted.

(Case where Processing Unit of Patrol Operation is 1 WL/2 BNK)

The processing order of the patrol operation in a case where theprocessing unit of the patrol operation is 1 WL/2 BNK is the same in thesecond embodiment and the first embodiment, for example.

Hereinafter, an example of the operation timing of the patrol operationin the case where the processing unit of the patrol operation is 1 WL/2BNK in the memory system 1 according to the second embodiment will bedescribed.

FIG. 23 shows an example of a timing chart of the patrol operation inthe case where the processing unit of the patrol operation is 1 WL/2 BNKin the second embodiment. As shown in FIG. 23, the memory system 1executes the operation corresponding to the bank BNK0 and the operationcorresponding to the bank BNK1 in parallel, as appropriate.

Specifically, first, the memory controller 20 sequentially transmits thecommand CMD corresponding to the tracking read of the lower page to thebanks BNK0 and BNK1. Upon receiving the command CMD corresponding to thetracking read of the lower page, each of the banks BNK0 and BNK1 startsthe tracking read of the lower page.

At this time, the tracking read in the bank BNK0 and the tracking readin the bank BNK1 can be executed in parallel. The tracking read of thelower page is completed in the order of the bank BNK0 and the bank BNK1.

When the tracking read of the lower page in the bank BNK0 is completed,the memory controller 20 causes the bank BNK0 to output the read dataDout of the lower page (patrol operation of the lower page in BNK0).

Upon receiving the read data Dout of the lower page from the bank BNK0,the memory controller 20 transmits the command CMD corresponding to thetracking read of the middle page to the bank BNK0, and the bank BNK0starts the tracking read of the middle page.

When the bank BNK0 is executing the tracking read of the middle page,the memory controller 20 causes the bank BNK1 to output the read dataDout of the lower page (patrol operation of the lower page in BNK1).

Upon receiving the read data Dout of the lower page from the bank BNK1,the memory controller 20 transmits the command CMD corresponding to thetracking read of the middle page to the bank BNK1, and the bank BNK1starts the tracking read of the middle page.

Thereafter, similarly, the tracking read in the bank BNK0 and output ofread data in the bank BNK1 can be executed in parallel as appropriate,and the tracking read in the bank BNK1 and output of read data in thebank BNK0 can be executed in parallel as appropriate.

In the second embodiment, when the processing unit of the patroloperation is 1 WL/2 BNK, the memory controller 20 cannot interrupt thehost read until all of the 1 WL/2 BNK processing is completed.

[2-2] Effects of Second Embodiment

As described above, in the patrol operation in the memory system 1according to the second embodiment, the defect detection is executedusing the read result in the tracking read. That is, in the patroloperation in the memory system 1 according to the second embodiment, thehistory learning and the defect detection can be executed by a singleoperation of the NAND-type flash memory 10.

Consequently, in the memory system 1 according to the second embodiment,since the shift read described in the first embodiment can be omitted,the time for the entire patrol operation can be shortened. Accordingly,the memory system 1 according to the second embodiment can speed up thepatrol operation as compared to the first embodiment.

In the first and second embodiments described above, although the secondembodiment looks superior to the first embodiment, which one is superiordepends on the command processing unit in the memory system 1.

For example, when the command processing unit is large like the 1 WL/2BNK unit, if the host read and the patrol operation conflict, theinstruction of the host read is kept waiting until the patrol operationcorresponding to 1 WL/2 BNK is completed.

In this case, since the parallel operation between the banks BNK ispossible in both the first and second embodiments, the processing timeof the patrol operation is longer in the first embodiment executing theshift read additionally. However, when the command processing unit islarge, it is not possible to separate the history learning and thedefect detection, so that the second embodiment tends to have a longerwaiting time when the host read and the patrol operation conflict.

In order to shorten the waiting time, it is conceivable to make thecommand processing unit in the memory system 1 as small as the 1 page/1BNK unit. When the command processing unit becomes small, since thehistory learning and the defect detection are completed in a short time,the latency is improved.

On the other hand, when the command processing unit is small, in thememory system 1 according to the second embodiment, the read result ofthe tracking read disappears due to interruption of the host read. Thatis, in the memory system 1 according to the second embodiment, thetracking read cannot be executed in parallel between the banks BNK, andthe processing time of the tracking read becomes long.

On the other hand, in the memory system 1 according to the firstembodiment, as described in the variation, the history learning can beprocessed in parallel between the banks BNK, and the defect detectioncan be processed in units of the bank BNK. As a result, the memorysystem 1 according to the first embodiment can process tracking readwith a long processing time in parallel, so that the processing time ofthe tracking read can be made shorter than in the second embodiment.

As described above, the memory system 1 can appropriately change thecommand processing unit and appropriately selectively use the first andsecond embodiments to execute optimum patrol operation according to thepurpose of the SSD.

[3] Third Embodiment

A memory system 1 according to a third embodiment has the sameconfiguration as the memory system 1 according to the first embodiment,for example. In the memory system 1 according to the third embodiment,in each patrol period, the patrol operation is appropriately thinned outaccording to an address of the word line WL. Hereinafter, points of thememory system 1 according to the third embodiment different from thefirst and second embodiments will be described.

[3-1] Patrol Operation

FIG. 24 shows an example of an execution cycle of the patrol operationin the memory system 1 according to a comparative example of the thirdembodiment. In a timing chart referred to in the following description,a period corresponding to one week from a predetermined date isextracted and shown. The word line WL and a block BLK which are targetsof the patrol operation are appropriately omitted. The vertical barshown in each item indicates that the patrol operation is beingexecuted.

In the description of the execution cycle of the patrol operation, it isassumed that the patrol period of the memory system 1 is set to one day.That is, in the following description, for example, the patrol operationcan be executed for all the word lines WL included in all the blocks BLKin the NAND-type flash memory 10 every one day.

As shown in FIG. 24, in the memory system 1 according to the comparativeexample of the third embodiment, in the patrol period of the first cycle(“1 day”), first, the patrol operation for the word lines WL0 to WL15 ofthe block BLK0 is sequentially executed, and the patrol operation forthe word lines WL0 to WL15 of the block BLK1 is subsequentlysequentially executed.

Thereafter, similarly, the patrol operation is sequentially executed forthe block BLK for which the patrol operation is not executed. Then, inthe memory system 1 according to the comparative example of the thirdembodiment, in the patrol periods of the second cycle (“2 day”) and thesubsequent cycles, the patrol operation is executed similarly to thefirst cycle.

FIG. 25 shows an example of an execution cycle of the patrol operationin the memory system 1 according to the third embodiment. As shown inFIG. 25, in the memory system 1 according to the third embodiment, inthe patrol period of the first cycle (“1 day”), as in the comparativeexample of the third embodiment, first, the patrol operation for theword lines WL0 to WL15 of the block BLK0 is sequentially executed, andthe patrol operation for the word lines WL0 to WL15 of the block BLK1 issubsequently sequentially executed. Thereafter, similarly, the patroloperation is sequentially executed for the block BLK for which thepatrol operation is not executed.

In the memory system 1 according to the third embodiment, for example,in each of the patrol periods of the second cycle (“2 day”) and thethird cycle (“3 day”), among all the word lines WL to be subjected tothe patrol operation, the patrol operation for the word line WL otherthan the specific word lines WL is thinned out.

As the specific word lines WL, for example, the word lines WL0 and WL15corresponding to the memory cell transistor MT located at an end of theNAND string NS and the word lines WL7 and WL8 corresponding to thememory cell transistor MT adjacent to a joint JT of a memory pillar MPare set.

In such a case, in each of the patrol periods of the second and thirdcycles, the patrol operation for each of the word lines WL1 to WL6 andWL9 to WL14 is omitted, and the patrol operation for the other wordlines WL is executed.

Then, in the memory system 1 according to the third embodiment, in thepatrol periods of the fourth cycle (“4 day”) and the subsequent cycles,the cycle of the patrol operation is repeated similarly to the first tothird cycles. That is, in the memory system 1 according to the thirdembodiment, the patrol period during which the patrol operation isthinned out is appropriately inserted in a repeated patrol period.

[3-2] Effects of Third Embodiment

When the memory pillar MP has a plurality of pillars connected in the Zdirection, it can be inferred that reliability of data stored in thememory cell transistor MT varies according to the address of the wordline WL.

For example, the memory cell transistor MT corresponding to the end ofthe NAND string NS and the memory cell transistor MT approaching thejoint JT have larger variations in characteristics than those of thememory cell transistors MT arranged in the other portions, and it can beinferred that the data retention characteristics are inferior.

In such a case, it is conceivable that the effect of executing thepatrol operation is higher in the memory cell transistor MTcorresponding to the end of the NAND string NS or approaching the jointJT than in the memory cell transistor MT disposed in another portion.

As a storage capacity of the memory system 1 increases, the number ofobjects to be subjected to the patrol operation increases, and the timeduring which the memory system 1 is executing the patrol operationbecomes long. In such a case, there is a high possibility that the hostread and the patrol operation will conflict, and the latency of thememory system 1 may be degraded.

Thus, in the memory system 1 according to the third embodiment, thepatrol period during which the patrol operation is thinned out isappropriately inserted with respect to a repeated patrol period. In thepatrol period during which the patrol operation is thinned out, thepatrol operation is selectively executed for the word line WL highlyeffective for executing the patrol operation.

In other words, in the memory system 1 according to the thirdembodiment, a cycle of the patrol operation is optimized according toreliability of the word line WL. By optimizing the patrol operationcycle according to the reliability, the reliability of data ismaintained also in the word line WL which is an object of thinning-outof the patrol operation.

Consequently, the memory system 1 according to the third embodiment canguarantee the reliability of data to be stored, as in the case where thepatrol operation for all the word lines WL is executed as in thecomparative example of the third embodiment. Since the memory system 1according to the third embodiment can reduce a processing amount of thepatrol operation, the probability of the conflict between the host readand the patrol operation can be reduced, and the latency can beimproved.

In the above description, the case where the single memory pillar MP hasa plurality of pillars connected in the Z direction has beenexemplified. However, the operation described in the third embodiment isalso applicable to a case where the memory pillar MP has anotherstructure. In the patrol operation in the third embodiment, it issufficient that the patrol operation cycle is optimized at leastaccording to the reliability of the word line WL.

[4] Fourth Embodiment

A memory system 1 according to a fourth embodiment has the sameconfiguration as the memory system 1 according to the first embodiment,for example. The memory system 1 according to the fourth embodimentchanges an execution frequency of patrol operation in accordance with alapse of time after data is written in a block BLK. Hereinafter, pointsof the memory system 1 according to the fourth embodiment different fromthe first to third embodiments will be described.

[4-1] Patrol Operation

FIG. 26 shows an example of a change in read voltage after writing to amemory cell transistor MT in the memory system 1 according to the fourthembodiment. In a graph shown in FIG. 26, the vertical axis correspondsto a correction value (DAC value) of the read voltage, and thehorizontal axis corresponds to a retention time indicating an elapsedtime after data is written in the memory cell transistor MT. As shown inFIG. 26, each read voltage changes based on the elapsed time (dataretention time) after data is written in the memory cell transistor MT.

Specifically, for example, the amount of change in each read voltageimmediately after writing is larger than the amount of change in eachread voltage after a predetermined time has elapsed since writing. Inother words, the amount of change in read voltage is large for a whilefrom immediately after writing, and when the time elapses to someextent, the amount of change in read voltage becomes small. Thus, thememory system 1 according to the fourth embodiment changes an executioncycle of the patrol operation after data is written, in accordance withthe lapse of time after the data is written.

FIG. 27 shows an example of the execution cycle of the patrol operationin the memory system 1 according to the fourth embodiment. In a timingchart referred to in the following description, the vertical bar shownin each item indicates that the patrol operation is executed for allword lines WL in the block BLK, for example. It is assumed that writeoperation for each of the illustrated blocks BLK0 to BLK7 has beenexecuted on the same day. The illustrated period corresponds to thenumber of days elapsed since data has been written in each of the blocksBLK.

As shown in FIG. 27, in the memory system 1 according to the fourthembodiment, the patrol operation is executed sequentially from the blockBLK0 in a patrol period of the first cycle (“1 day”). Then, the samepatrol operation as in the first cycle is executed for each of thepatrol periods of the second cycle (“2 day”), the third cycle (“3 day”),the fifth cycle (“5 day”), and the seventh cycle (“7 day”).

On the other hand, in the memory system 1 according to the fourthembodiment, the patrol operation is omitted in the patrol periods of,for example, the fourth cycle (“4 day”) and the sixth cycle (“6 day”).That is, the patrol operation is executed every day for two daysimmediately after writing, and the patrol operation is omitted onceevery two days since third day (“thinning-out target cycle”).

In the memory system 1 according to the fourth embodiment, it ispossible to appropriately change the frequency of inserting thethinning-out target cycle. The thinning-out target cycle is inserted atleast after a predetermined number of patrol periods are repeated fromimmediately after writing. The thinning-out target cycle may be insertedcontinuously.

In the above description, the case where the thinning-out target cycleis inserted in the repeated patrol periods is exemplified, but thepresent invention is not limited thereto. For example, in the operationdescribed in the fourth embodiment, the cycle in which the patroloperation is executed may be regarded as being changed according to thelapse of time after writing. In this case, the cycle in which the patroloperation is executed is set so as to be long with the lapse of time.

[4-2] Effects of Fourth Embodiment

As described with reference to FIG. 26 in the fourth embodiment, theamount of change in read voltage changes with the lapse of time. Theamount of change in read voltage is the largest after writing anddecreases with the lapse of time. Such a phenomenon may be caused by aparticularly large drop in a threshold voltage of the memory celltransistor MT immediately after writing.

Thus, the memory system 1 according to the fourth embodiment changes thecycle in which the patrol operation is executed, according to the lapseof time after writing. In other words, the memory system 1 according tothe fourth embodiment executes the patrol operation at an appropriatefrequency according to a change in data retention characteristics of thememory cell transistor MT.

Consequently, the memory system 1 according to the fourth embodiment canreduce a processing amount of the patrol operation while guaranteeingreliability of data to be stored. Accordingly, the memory system 1according to the fourth embodiment can reduce the probability of theconflict between host read and the patrol operation and can improve thelatency.

[5] Fifth Embodiment

A memory system 1 according to a fifth embodiment has the sameconfiguration as the memory system 1 according to the first embodiment,for example. The memory system 1 according to the fifth embodimentinserts shortened patrol operation in accordance with a lapse of timeafter data is written in a block BLK. Hereinafter, points of the memorysystem 1 according to the fifth embodiment different from the first tofourth embodiments will be described.

[5-1] Abbreviated Tracking Read

In the shortened patrol operation, the memory system 1 executesabbreviated tracking read. In the abbreviated tracking read, trackingfor estimating a correction value of read voltage is executed only forhigher read voltage, for example. A correction value of read voltagecorresponding to the lower read voltage is estimated from the correctionvalue of the higher read voltage.

FIG. 28 shows an example of a tracking target state in the abbreviatedtracking read of the memory system 1 according to the fifth embodiment.As shown in FIG. 28, in this example, when an object to be read is amiddle page, tracking corresponding to a read voltage FR is executed,and tracking corresponding to each of read voltages BR and DR isomitted. When the object to be read is an upper page, trackingcorresponding to a read voltage GR is executed, and trackingcorresponding to a read voltage CR is omitted.

On the other hand, when the object to be read is a lower page, trackingcan be executed for both read voltages AR and ER, for example. Thereason for this is that a read error tends to occur between thresholddistribution at “A” state corresponding to the lowest write state andthreshold distribution at “ER” state. That is, in the abbreviatedtracking read, a plurality of read voltages may be used in a pageincluding read voltage corresponding to the lowest write state.

FIG. 29 is a timing chart showing an example of the abbreviated trackingread in the memory system 1 according to the fifth embodiment and showsan example of the abbreviated tracking read corresponding to readoperation of the upper page. As shown in FIG. 29, when the abbreviatedtracking read of the upper page is executed, a memory controller 20transmits, for example, a command “xzh”, a command “03h”, a command“00h”, address information “ADD” of five cycles, and a command “30h” toa NAND-type flash memory 10 in this order.

The command “xzh” is a command instructing the NAND-type flash memory 10to execute abbreviated tracking read. Upon receiving the command “30h”,the NAND-type flash memory 10 transitions from a ready state to a busystate, and a sequencer 15 starts abbreviated tracking read.

When the abbreviated tracking read of the upper page starts, forexample, tracking voltages GRt1, GRt2, GRt3, GRt4, and GRt5corresponding to the read voltage GR are sequentially applied to aselected word line WLsel.

The sequencer 15 asserts the control signal STB while each trackingvoltage is being applied to the selected word line WLsel. Then, thesequencer 15 estimates an optimum read voltage GRc based on read resultsof the tracking voltages GRt1 to GRt5. Further, the sequencer 15estimates an optimum read voltage CRc based on the optimum read voltageGRc.

A correction value corresponding to the optimum read voltage obtained bythe abbreviated tracking read is held in a status register 13A, forexample, in the same manner as normal tracking read. Thereafter, theoptimum read voltages CRc and GRc are sequentially applied to theselected word line WLsel. Since the operation of the abbreviatedtracking read thereafter is the same as the operation of the trackingread described with reference to FIG. 13, the description thereof willbe omitted.

In this example, the memory system 1 can also execute abbreviatedtracking read of other pages in the same manner as the abbreviatedtracking read of the upper page. For example, in the tracking read ofthe middle page, a command “02h” is used instead of the command “03h”.In the abbreviated tracking read of the middle page, the trackingvoltage and the read voltage to be used are appropriately changed. Sincethe other operations in the abbreviated tracking read of the middle pageare the same as those in the abbreviated tracking read of the upperpage, their explanations are omitted.

[5-2] Patrol Operation

Referring again to FIG. 26, a change in each read voltage based on anelapsed time (data retention time) after data is written in the memorycell transistor MT will be described. As shown in FIG. 26, the amount ofchange in each read voltage greatly differs between a group having alower read voltage and a group having a higher read voltage.

Specifically, for example, the amount of change in read voltage in eachof the read voltages BR, CR, and DR is smaller than the amount of changein read voltage in each of the read voltages ER, FR, and GR. In otherwords, the amount of change in read voltage tends to be larger in thegroup having a higher read voltage (read voltages ER, FR, and GR) thanin the group having a lower read voltage (read voltages BR, CR, and DR).

Thus, the memory system 1 according to the fifth embodimentappropriately inserts the shortened patrol operation using theabbreviated tracking read in accordance with a lapse of time after datais written. In the abbreviated tracking read, tracking for read voltagewith a relatively large amount of change in read voltage is executed,and tracking for read voltage with a relatively small amount of changein read voltage is omitted.

FIG. 30 shows an example of an execution cycle of the patrol operationin the memory system 1 according to the fifth embodiment. As shown inFIG. 30, in the memory system 1 according to the fifth embodiment, thepatrol operation is executed sequentially from a block BLK0 in a patrolperiod of the first cycle (“1 day”). Then, the same patrol operation asin the first cycle is executed for each of the patrol periods of thesecond cycle (“2 day”), the fourth cycle (“4 day”), and the seventhcycle (“7 day”).

On the other hand, in the memory system 1 according to the fifthembodiment, the shortened patrol operation is executed sequentially fromthe block BLK0 in the patrol periods of, for example, the third cycle(“3 day”) the fifth cycle (“5 day”), and the sixth cycle (“6 day”)(“shortening target cycle”). That is, normal patrol operation isexecuted every day for two days immediately after writing, and theshortened patrol operation is appropriately inserted since third day(“thinning-out target cycle”).

In the memory system 1 according to the fifth embodiment, it is possibleto appropriately change the frequency of inserting the shortening targetcycle. The shortening target cycle is inserted at least after apredetermined number of patrol periods are repeated from immediatelyafter writing. The shortening target cycle may be inserted continuously.

[5-3] Effects of Fifth Embodiment

As described with reference to FIG. 26 in the fifth embodiment, theamount of change in read voltage is different for each read voltage. Theamount of change in read voltage tends to increase as the read voltageincreases. Such a phenomenon may be caused by the fact that the higherthe threshold voltage, the larger a drop in a threshold voltage of thememory cell transistor MT immediately after writing.

Thus, the memory system 1 according to the fifth embodimentappropriately uses the normal patrol operation with high accuracy andthe high-speed shortened patrol operation according to the lapse of timeafter writing. Specifically, the memory system 1 according to the fifthembodiment executes the normal patrol operation with high accuracyduring a period in which the amount of change in read voltage is large,that is, in a predetermined period including immediately after writing.On the other hand, the memory system 1 according to the fifth embodimentappropriately executes the shortened patrol operation during a period inwhich the amount of change in read voltage is small, that is, after apredetermined time has elapsed.

The accuracy of the correction value of the read voltage obtained by theshortened patrol operation is inferior to the normal patrol operation.However, since the memory system 1 according to the fifth embodimentalso executes the normal patrol operation as appropriate, the accuracyof the correction value corresponding to the read voltage for whichtracking is omitted in the shortened patrol operation can also beguaranteed, and reliability of read data can be maintained.

As a result, the memory system 1 according to the fifth embodiment canreduce a processing amount of the patrol operation while guaranteeingthe reliability of data to be stored. Accordingly, the memory system 1according to the fifth embodiment can reduce a waiting time when hostread and the patrol operation conflict and can improve the latency.

[6] Sixth Embodiment

A memory system 1 according to a sixth embodiment has the sameconfiguration as the memory system 1 according to the first embodiment,for example. The memory system 1 according to the sixth embodimentselectively executes patrol operation in accordance with an executiondata retention time after data is written in a block BLK. Hereinafter,points of the memory system 1 according to the sixth embodimentdifferent from the first to fifth embodiments will be described.

[6-1] Patrol Operation

In the memory system 1 according to the sixth embodiment, in the patroloperation, the system temperature of the memory system 1 measured by atemperature sensor 25 or temperature information acquired from aNAND-type flash memory 10 is used. For example, a CPU 21 of a memorycontroller 20 updates a patrol management parameter based on ameasurement result of the system temperature of the memory system 1 bythe temperature sensor 25.

The patrol management parameter is held in the RAM 22, for example. Inthe sixth embodiment, information on the execution data retention timefor each of the blocks BLK is recorded in the patrol managementparameter.

The execution data retention time is a time obtained by converting atime during which the memory system 1 has been heated under varioustemperature conditions after data has been written in the block BLK intoa time during which the memory system 1 has been heated under a specifictemperature condition. For example, an elapsed time since data iswritten in the block BLK is measured by a timer 28, and the executiondata retention time is calculated based on a measurement result of thetemperature sensor 25 for each predetermined cycle.

A coefficient used when converting into time under a specifictemperature condition is set based on the number of days when errorcorrection of read data becomes impossible when the memory system 1 isheated under each temperature condition, for example. Hereinafter, it isassumed that this specific temperature is set at 40° C. The temperatureused as a criterion of the execution data retention time is not limitedto 40° C. and can be appropriately changed.

FIG. 31 shows an example of a temperature dependence of data retentioncharacteristics and a patrol criteria after writing in the memory celltransistor MT in the memory system 1 according to the sixth embodiment.In the graph shown in FIG. 31, the vertical axis corresponds to aneffective time @ 40° C. (Day) after data is retained, that is, theeffective data retention time based on 40° C., and the horizontal axiscorresponds to a heating time (Day) for the memory cell transistor MT.As shown in FIG. 31, the effective data retention time increases as thesystem temperature of the memory system 1 increases.

Specifically, an effective time when the memory system 1 is heated forone day at a system temperature of 40° C., for example, is calculated asone day. In this case, an effective time when the memory system 1 isheated for one day at a system temperature of 25° C. is shorter than oneday, and an effective time when the memory system 1 is heated for oneday at a system temperature of 70° C. is longer than one day.

An effective time during which reliability of data stored in the blockBLK can be maintained is set to, for example, one month. For the blockBLK in which the effective time exceeds one month, it is assumed thaterror correction becomes difficult, and it is preferable to execute thepatrol operation.

Thus, the memory system 1 according to the sixth embodiment selectivelyexecutes the patrol operation based on the effective data retention timerecorded in the patrol management parameter.

FIG. 32 shows an example of the patrol management parameter in thepatrol operation of the memory system 1 according to the sixthembodiment. As shown in FIG. 32, the effective data retention time foreach of the blocks BLK is recorded in the patrol management parameter inthe sixth embodiment. The effective data retention time recorded in thepatrol management parameter may be updated sequentially or may beupdated at predetermined intervals.

When the memory system 1 according to the sixth embodiment detects theblock BLK in which the effective data retention time exceeds apredetermined criterion, the memory system 1 executes the patroloperation for the block BLK. When the criteria of the patrol operationis set to “the effective time exceeds one month”, in this example,blocks BLK5 and BLK6 exceed the criteria. That is, once the effectivedata retention time exceeds one month, the memory system 1 executes thepatrol operation for each of the blocks BLK5 and BLK6.

In this example, the effective data retention time is continuouslyupdated after the patrol operation is executed. In this case, after thepatrol operation is executed, if the time set as the criteria of thepatrol operation elapses again, the patrol operation is executed again.The number of executions of the patrol operation may be recorded in thepatrol management parameter, and the patrol operation can be executedbased on the execution number and the criteria of the patrol operation.

After the patrol operation is executed, the effective data retentiontime corresponding to the relevant block BLK may be reset. Even in sucha case, the memory system 1 according to the sixth embodiment canselectively execute the patrol operation based on the effective dataretention time for each of the blocks BLK.

[6-2] Effects of Sixth Embodiment

As described with reference to FIG. 31 in the sixth embodiment, the dataretention characteristics of the memory cell transistor MT deteriorateas the system temperature increases and tend to improve as the systemtemperature decreases. For example, even in the memory system 1 in whichthe limit of maintaining the reliability of data is one day when thememory system 1 has a system temperature of 70° C., if the systemtemperature is 40° C., the reliability of data can be maintained for onemonth, for example.

Thus, when the system temperature of the memory system 1 is high, it ispreferable that the frequency of the patrol operation is higher. On theother hand, when the system temperature of the memory system 1 is low,problems are unlikely to occur even if the frequency of the patroloperation is low.

Thus, the memory system 1 according to the sixth embodiment changes thefrequency with which the patrol operation is executed based on theeffective data retention time according to the system temperature.Specifically, in the memory system 1 according to the sixth embodiment,appropriately with reference to the measurement result of thetemperature sensor 25, the measurement result is converted into theeffective data retention time under a specific temperature condition,for example. In the memory system according to the sixth embodiment, apredetermined criterion is provided for the effective data retentiontime, and the patrol operation is executed at an appropriate frequencybased on the criterion.

Consequently, the memory system 1 according to the sixth embodiment canreduce a processing amount of the patrol operation while guaranteeingthe reliability of data to be stored. Accordingly, the memory system 1according to the sixth embodiment can reduce the probability of theconflict between host read and the patrol operation and can improve thelatency. Note that, the effects described above in the sixth embodimentare not always the case. For example, the frequency of the patroloperation increases as the system temperature increases. In this case,the effects described above in the sixth embodiment may not be apparent.

[7] Seventh Embodiment

A memory system 1 according to a seventh embodiment has the sameconfiguration as the memory system 1 according to the first embodiment,for example. In the memory system 1 according to the seventh embodiment,a block BLK retaining valid data is set as a patrol target, and patroloperation is selectively executed. Hereinafter, points of the memorysystem 1 according to the seventh embodiment different from the first tosixth embodiments will be described.

[7-1] Patrol Operation

FIG. 33 shows an example of a patrol management parameter in the patroloperation of the memory system 1 according to the seventh embodiment. Asshown in FIG. 33, in the patrol management parameter in the seventhembodiment, for example, a valid data area flag is recorded for each ofthe blocks BLK. The valid data area flag indicates whether or not validdata is written in the relevant block BLK.

For example, when the valid data area flag is “TRUE”, this indicatesthat the valid data is written in this block BLK. On the other hand,when the valid data area flag is “FALSE”, this indicates that onlyinvalid data is written in this block BLK, or the block BLK is in anerased state. Then, the memory system 1 according to the seventhembodiment executes the patrol operation based on the valid data areaflag.

FIG. 34 shows an example of an execution cycle of the patrol operationin the memory system 1 according to the seventh embodiment. In thisexample, it is assumed that the valid data area flag corresponding toblocks BLK2, BLK3, and BLK4 is “TRUE”, and the valid data area flagcorresponding to the other blocks BLK is “FALSE”.

As shown in FIG. 34, in the memory system 1 according to the seventhembodiment, in each patrol period, the patrol operation for the blocksBLK2, BLK3, and BLK4 which are the written blocks BLK is executed. Onthe other hand, in each patrol period, the patrol operation is notexecuted for the other blocks BLK in which the valid data area flag is“FALSE”.

As described above, the memory system 1 according to the seventhembodiment can selectively execute the patrol operation for the blockBLK in which the valid data is written, based on the valid data areaflag.

In the operation described above, it can be rephrased as “the memorycontroller 20 reads a logical-physical conversion table from a systemmanagement area (for example, the RAM 22), executes the patrol operationfor a block retaining valid data, and omits the patrol operation fordata that has become invalid due to overwriting and an erased block”.

[7-2] Effects of Seventh Embodiment

In the memory system 1, the block BLK including valid data and the blockBLK including only invalid data coexist. The block BLK including onlyinvalid data corresponds to, for example, the block BLK in the erasedstate in which writing is not executed and the block BLK in a copysource in garbage collection or the like.

Read operation is executed for the block BLK including valid data. Onthe other hand, the read operation is not executed for the block BLKincluding only invalid data. Thus, even if the patrol operation for theblock BLK including only invalid data is omitted, no problem arises.

Thus, the memory system 1 according to the seventh embodimentselectively executes the patrol operation for the block BLK includingvalid data and omits the patrol operation for the block BLK includingonly invalid data.

Consequently, the memory system 1 according to the seventh embodimentcan reduce a processing amount of the patrol operation. Accordingly, thememory system 1 according to the seventh embodiment can reduce theprobability of the conflict between host read and the patrol operationand can improve the latency.

[8] Other Variations and the Like

The memory system of the embodiment <for example, FIG. 1, 1> includes asemiconductor memory and a memory controller. A semiconductor memory<for example, FIG. 3, 10> includes a plurality of memory cells connectedin series and a plurality of word lines. Each of the plurality of wordlines is connected to each of the memory cells. A memory controller <forexample, FIG. 1, 20> executes the patrol operation including readoperation of a semiconductor memory. The word lines are classified intoone of a first group and a second group based on the address of the wordline. The memory controller executes a plurality of the patroloperations in which the word lines are selected in a first patrolperiod. In a second patrol period <for example, FIG. 25, thinning-outtarget cycle> subsequent to the first patrol period, the memorycontroller executes the patrol operation in which the word line includedin the first group is selected and omits the patrol operation in whichthe word line included in the second group is selected. Consequently,the memory system of the embodiment can guarantee reliability of dataand improve latency.

The above embodiments can be combined as appropriate. For example, anyone of the third to seventh embodiments may be applied to the first orsecond embodiment. The third to seventh embodiments can be combined witheach other.

In the above embodiments, the data assignment to the memory celltransistor MT can be appropriately changed. For example, when 3-bit datais stored in the memory cell transistor MT, the data assignment otherthan the 2-3-2 code may be applied.

In the above embodiment, the case where the 3-bit data is stored in thesingle memory cell transistor MT has been exemplified. However, data of1 bit, 2 bits, or 4 bits or more may be stored in the single memory celltransistor MT. Also in such a case, the memory system 1 can execute theoperation described in the above embodiments.

In each of the tracking read, the shift read, and the abbreviatedtracking read described in the above embodiments, the voltages appliedto the selected word line WLsel are the same as the voltage of thesignal line CG transferring the voltage to the row decoder module 18,for example.

That is, the voltage applied to various wires or the period during whichthe voltage is applied can roughly be known by examining the voltage ofthe corresponding signal line CG. In estimating the voltage of the wordline WL from the voltage of the signal line CG, a voltage drop due tothe transistor TR included in the row decoder RD may be taken intoconsideration. In this case, the voltage of the word line WL becomeslower than the voltage applied to the corresponding signal line CG bythe voltage drop of the transistor TR.

In the above embodiments, each of the commands “xxh”, “yyh”, “zzh”, and“zxh” used in the description can be replaced by any command.

In the above embodiments, the case where the commands “01h” to “03h” areused as the commands instructing the operations corresponding to thefirst to third pages, respectively, has been described as an example,but the present invention is not limited thereto. For example, thecommands “01h” to “03h” may be replaced by other commands, or thesecommands may be omitted by including information on the page in theaddress information ADD.

In the above embodiment, the case where the memory cell transistors MTprovided in the memory cell array 11 are three-dimensionally stacked hasbeen described as an example, but the present invention is not limitedthereto. For example, the configuration of the memory cell array 11 maybe a planar NAND-type flash memory in which the memory cell transistorsMT are two-dimensionally arranged. Even in such a case, the aboveembodiment can be realized, and similar effects can be obtained.

The memory cell array 11 in the above embodiment may have otherconfigurations. The other configurations of the memory cell array 11 aredescribed in, for example, Specification of U.S. patent applicationPublication Ser. No. 12/407,403 filed on Mar. 19, 2009, titled “THREEDIMENSIONAL STACKED NONVOLATILE SEMICONDUCTOR MEMORY”, Specification ofU.S. patent application Publication Ser. No. 12/406,524 filed on Mar.18, 2009, titled “THREE DIMENSIONAL STACKED NONVOLATILE SEMICONDUCTORMEMORY”, Specification of U.S. patent application Publication Ser. No.12/679,991 filed on Mar. 25, 2010, titled “NON-VOLATILE SEMICONDUCTORSTORAGE DEVICE AND METHOD OF MANUFACTURING THE SAME”, and Specificationof U.S. patent application Publication Ser. No. 12/532,030 filed on Mar.23, 2009, titled “SEMICONDUCTOR MEMORY AND METHOD FOR MANUFACTURINGSAME”. The above patent applications are incorporated by referenceherein in their entirety.

The “connection” in this specification means electrical connection anddoes not exclude the fact that another element is interposed in theconnection. In the present specification, the “off state” means that avoltage less than the threshold voltage of the corresponding transistoris applied to a gate of the transistor, and, for example, it does notexclude the fact that a minute current such as a leak current of thetransistor flows.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

The invention claimed is:
 1. A memory system comprising: a plurality ofsemiconductor memories, each of which comprises a memory cell and a wordline connected to the memory cell and each of which is connected to acommon bus; and a memory controller configured to: execute a patroloperation, comprising creation of a correction value of read voltage inthe semiconductor memory and defect detection of the word line, inparallel to the semiconductor memories; and execute the patrol byoperation by selecting the word line in a first period immediately afterdata is written in the memory cell and a second period having a samelength as the first period and following the first period, wherein anumber of patrol periods of a cycle included in the first period is lessthan a number of the patrol periods of the cycle included in the secondperiod, wherein in creating the correction value, after a plurality ofkinds of read voltages are applied to the word line, the correctionvalue of the read voltage is determined based on read results based onthe plurality of kinds of read voltages, in the defect detection, thememory controller determines whether there is a defect in the word linebased on a result of a read operation to which the correction value isapplied, and the memory controller executes the defect detection basedon a result of a read operation executed subsequent to the creation ofthe correction value or based on a result of a read operation executedindependently of the correction value creation.
 2. The memory system ofclaim 1, wherein the memory controller causes the semiconductor memoryto output the correction value to the memory controller after creatingthe correction value, and uses the output correction value to instructthe semiconductor memory to perform read operation corresponding to thedefect detection.
 3. The memory system of claim 1, wherein the memorycontroller causes the semiconductor memory to output the correctionvalue to the memory controller after creating the correction value, thenoutput the result of the read operation executed continuously tocreation of the correction value to the memory controller, and executesthe defect detection using the output read result.
 4. The memory systemof claim 1, wherein the memory cell is capable of storing data of aplurality of bits including a first bit, data of the first bit isdetermined by read operation using a first read voltage and a secondread voltage higher than the first read voltage, the memory controllerexecutes the patrol operation in each of repeated patrol periods, therepeated patrol period is classified into either a first cycle or asecond cycle, the memory controller causes the semiconductor memory toexecute creation of the correction value selecting the first bit in eachof the patrol periods of the first cycle and the second cycle, aplurality of kinds of read voltages corresponding to the first readvoltage and a plurality of kinds of read voltages corresponding to thesecond read voltage are sequentially applied to the word line in thecorrection value creation of the first cycle, in creating the correctionvalue of the second cycle, the plurality of kinds of read voltagescorresponding to the second read voltage are applied to the word line,and application of the plurality of kinds of read voltages correspondingto the first read voltage is omitted.
 5. The memory system of claim 1,further comprising a plurality of blocks each comprising a plurality ofthe memory cells, wherein the memory controller reads a logical-physicalconversion table from a system management area, executes the patroloperation for a block retaining valid data, and omits the patroloperation for a block including invalid data or an erased block.
 6. Amemory system connectable to a host device comprising: a plurality ofsemiconductor memories, each of which comprises a memory cell and a wordline connected to the memory cell and each of which is connected to acommon bus; and a memory controller configured to repeat a patroloperation, comprising creation of a correction value of read voltage inthe semiconductor memory and defect detection of the word line, inparallel to the semiconductor memories, wherein in creating thecorrection value, after a plurality of kinds of read voltages areapplied to the word line, the correction value of the read voltage isdetermined based on read results based on the plurality of kinds of readvoltages, in the defect detection, the memory controller determineswhether there is a defect in the word line based on a result of a readoperation to which the correction value is applied, the memorycontroller executes the defect detection based on a result of a readoperation executed subsequent to the creation of the correction value orbased on a result of a read operation executed independently of thecorrection value creation, the memory controller is configured to usethe correction value of read voltage to read data of the memory cell, inread operation according to an instruction from the host device, thememory controller executes patrol operation selecting the word line in afirst period including immediately after data is written in the memorycell and a second period having the same length as the first period andfollowing the first period, and the number of the patrol periods of thesecond cycle included in the first period is smaller than the number ofthe patrol periods of the second cycle included in the second period. 7.The memory system of claim 6, wherein the memory controller causes thesemiconductor memory to output the correction value to the memorycontroller after creating the correction value, and uses the outputcorrection value to instruct the semiconductor memory to perform readoperation corresponding to the defect detection.
 8. The memory system ofclaim 6, wherein the memory controller causes the semiconductor memoryto output the correction value to the memory controller after creatingthe correction value, then output the result of the read operationexecuted continuously to creation of the correction value to the memorycontroller, and executes the defect detection using the output readresult.
 9. The memory system of claim 6, wherein the memory cell iscapable of storing data of a plurality of bits including a first bit,data of the first bit is determined by read operation using a first readvoltage and a second read voltage higher than the first read voltage,the memory controller executes the patrol operation in each of repeatedpatrol periods, the repeated patrol period is classified into either afirst cycle or a second cycle, the memory controller causes thesemiconductor memory to execute creation of the correction valueselecting the first bit in each of the patrol periods of the first cycleand the second cycle, a plurality of kinds of read voltagescorresponding to the first read voltage and a plurality of kinds of readvoltages corresponding to the second read voltage are sequentiallyapplied to the word line in the correction value creation of the firstcycle, in creating the correction value of the second cycle, theplurality of kinds of read voltages corresponding to the second readvoltage are applied to the word line, and application of the pluralityof kinds of read voltages corresponding to the first read voltage isomitted.
 10. The memory system of claim 6, wherein the memory controllerexecutes patrol operation selecting the word line in a first periodincluding immediately after data is written in the memory cell and asecond period having the same length as the first period and followingthe first period, and the number of the patrol periods of the secondcycle included in the first period is smaller than the number of thepatrol periods of the second cycle included in the second period. 11.The memory system of claim 6, further comprising a plurality of blockseach comprising a plurality of the memory cells, wherein the memorycontroller reads a logical-physical conversion table from a systemmanagement area, executes the patrol operation for a block retainingvalid data, and omits the patrol operation for a block including invaliddata or an erased block.
 12. A memory system comprising: a plurality ofsemiconductor memories, each of which comprises a memory cell and a wordline connected to the memory cell and each of which is connected to acommon bus; a memory controller configured to execute a patroloperation, comprising creation of a correction value of read voltage inthe semiconductor memory and defect detection of the word line, inparallel to the semiconductor memories; and a temperature sensor,wherein in creating the correction value, after a plurality of kinds ofread voltages are applied to the word line, the correction value of theread voltage is determined based on read results based on the pluralityof kinds of read voltages, in the defect detection, the memorycontroller determines whether there is a defect in the word line basedon a result of a read operation to which the correction value isapplied, the memory controller executes the defect detection based on aresult of a read operation executed subsequent to the creation of thecorrection value or based on a result of a read operation executedindependently of the correction value creation, and the frequency withwhich the memory controller executes patrol operation selecting the wordline is higher in a case where a measured temperature of the temperaturesensor is a second temperature higher than a first temperature than in acase where the measured temperature of the temperature sensor is thefirst temperature.
 13. The memory system of claim 12, wherein the memorycontroller causes the semiconductor memory to output the correctionvalue to the memory controller after creating the correction value, anduses the output correction value to instruct the semiconductor memory toperform read operation corresponding to the defect detection.
 14. Thememory system of claim 12, wherein the memory controller causes thesemiconductor memory to output the correction value to the memorycontroller after creating the correction value, then output the resultof the read operation executed continuously to creation of thecorrection value to the memory controller, and executes the defectdetection using the output read result.
 15. The memory system of claim12, wherein the memory cell is capable of storing data of a plurality ofbits including a first bit, data of the first bit is determined by readoperation using a first read voltage and a second read voltage higherthan the first read voltage, the memory controller executes the patroloperation in each of repeated patrol periods, the repeated patrol periodis classified into either a first cycle or a second cycle, the memorycontroller causes the semiconductor memory to execute creation of thecorrection value selecting the first bit in each of the patrol periodsof the first cycle and the second cycle, a plurality of kinds of readvoltages corresponding to the first read voltage and a plurality ofkinds of read voltages corresponding to the second read voltage aresequentially applied to the word line in the correction value creationof the first cycle, in creating the correction value of the secondcycle, the plurality of kinds of read voltages corresponding to thesecond read voltage are applied to the word line, and application of theplurality of kinds of read voltages corresponding to the first readvoltage is omitted.
 16. The memory system of claim 12, wherein thememory controller executes patrol operation selecting the word line in afirst period including immediately after data is written in the memorycell and a second period having the same length as the first period andfollowing the first period, and the number of the patrol periods of thesecond cycle included in the first period is smaller than the number ofthe patrol periods of the second cycle included in the second period.17. The memory system of claim 12, further comprising a plurality ofblocks each comprising a plurality of the memory cells, wherein thememory controller reads a logical-physical conversion table from asystem management area, executes the patrol operation for a blockretaining valid data, and omits the patrol operation for a blockincluding invalid data or an erased block.